If i assume, which seems the target of Blizzard:

1. All classes fulfill there role on same degree, e.g. dps for all dps-specs are the same
2. The classes have not group-specific advantage over all other classes.

At the Blizzcon Blizzard revealed that there will be a dual-talent spec, which makes switching
the role in a group very easy and strong. The gear already became more standardized, e.g. nearly
same gear for caster/healer.

So, any group (and specially a raid) will favor this flexibility over the single-purpose classes, since
the flexibility allows easy composition of a group. Even in a battleground one person my simple switch
the role from dps to healer, if the group is in need of a healer (and they are not in fight.)

Hence, i will expect, that the single-propose classes will basically removed from large parts of the game.

These single-propose classes are:

• Hunter,
• Mage,
• Rogue and
• Warlock

I do not believe that Blizzard will give these classes skills that ensures to be in a group, because this contradicts
assumption #2, which Blizzard already utilized to remove the necessity for Curse of Elements from warlocks and
give a better ability to balance druids and unholy death knights.
(Blizzard had the change to do it and chose the other alternative.)

Do i miss something crucial?
It is time to roll away from this classes?

Thank you for this information. And yes, it's time to roll away from those classes. Roll right out of WoW, even. Cya!

Well, as long as they keep up with dps with the hybrids, the classes are not exactly dead. Just less useful then others. You won't need THAT many hybrids in a raid to cover all your raid setup needs.

Say if you have a basic setup of 8 healers, 3 tanks and 14 dps in a 25 men raid, you will have 2-3 palas, 2-3 priests, 2-3 druids and 2-3 shamans, with the number for most of these being rather 3 or maybe even 4 then 2

Just with those you can easily switch from the setup above to anything from 4 healers and 6 tanks to 12 healers/x, which are extremes that I very much doubt will ever be necessairy for anything.

But yeah, if dps actually matches, playing these classes will give you considerably less options and sooner or later the classes will start dieing out as people will slowly reroll to hybrids. I very much doubt devs will let that happen, it's a pretty blatant oversight and would have drastic results on class balance, which can't be their goal really.

You - and half of beta forums, as well - missed the part where single-purpose classes will still be doing higher DPS. It was said by Blizzard multiple times, but was always conveniently ignored. While currently it is supposed to be around 5%, they can always tweak those numbers, if people start stacking, say, Druids, for 10 Rebirths, or some other absurd example.
You also missed the fact, that respecs were easy and used in Sunwell, as well. It's just a matter of single portal and summon. With dual specs, you save maybe 5 minutes, which hardly makes or breaks a raid.

Hunter DPS is above the hybrids though Ret Paladins are pretty close, I'm not sure about the other pure classes, though I've not really raided with many pure dps classes in beta.

The problem will be more accentuated on 10mans rather than 25mans. Where the hybrids will shine, even if pure DPS classes do 5% more DPS a hybrid would be prefered cause of the utility they bring they can simply switch to tank/healing spec and help on some more tricky boss.

While obviously the classes listed will never fill a healer or maintank slot, it's kind of misleading to call them single-purpose as they're all classes with crowd control and other utility options. Taking for example hunters -- they feature threat management, CC and offtanking utility and a mana battery option. All of those 4 classes have abilities (some base, some talented) that are valuable in just about any raid even though the classes can only fit into a DPS slot.

Conversely not all hybrids will juggle specs to justify their raid role. Dual specs affects fewer raid slots than you seem to think. It allows for flexibility when a specific fight requires more healers than the rest (Sapphiron springs to mind) or when one fight has more stringent tanking requirements than the others. You hardly need to fill your raid with hybrids to satisfy those needs.

PS: Next up: topic about casuals vs. hardcore.

Okay, look at it this way if you want a less troll-ish answer.

The entire design base of WotLK and the buff changes/consolidations/what have you is because Blizzard wants you to bring people because they are good at this game not because their class has x, y, z capability, and that includes being able to tank or heal. With that in mind, and how malleable the game is in terms of being able to continually balance and patch things, assertions as those in the OP regarding "oh God, I'll need to re-roll to be more useful to the raid!" completely fail to grasp the underlying design decisions.

If things get out of hand, Blizzard will make adjustments, based on their stated goals. If you re-roll just because you feel a class is more useful and not because you enjoy playing that class more, Blizzard has failed in their goal. If Blizzard designs an instance which needs 10 healers 4 tanks on one fight and 6 healers 1 tank on another fight, they have failed in their goal. (And to a larger extent, say 3 tank, 3 healer in a 10-man fight succeeded by a 1 tank 2 healer fight with such strict dps requirements you absolutely need 7 people going balls out)

Barring EXTREME situations such as your entire 25-man raiding roster doesn't have a single shaman available for Bloodlust/Heroism and you haven't been able to recruit one for the past six months, raid comps are going to be a lot more flexible than they were in TBC, specifically Sunwell.

Just play your class, play it well. Let Blizzard worry about their job.

Just yesterday at Blizzcon someone brought this question up and the answer was that they realize this is an issue, since players playing pure classes don't have the luxury to keep their class but respec to a different raidrole if there's a bad time for them. So they're buffing their utility and always absolutely making sure they don't fall behind.

How effective they're going to be at that remains to be seen

I never really understood the fun of being a pure dps class in first place so I'm not really in the position to comment on how they are, are not or should be competetive comapred to hybrids.

However, I would like to point out:
BlizzCon 2008: WoW Q&A Panel Liveblog - WOW Insider
So, I suspect when comparing equally geared and equally skilled players, "the pure dps" will do more dps than "hybrid dps". Shouldn't that be enough?

However, the great player will do more damage than a good player, and good player will do more damage than the avarage player. I guess the main problem here what people have is that you don't have automatically guaranteed spot anymore over other classes. You know, some thing why that ass guild master/raid leader should take you instead of his personal favourites

What everyone else said.

There will never be a raid without DPSers. As long as you're always going to need at least 50% of your raid to be DPSers (even a Twins raid typically has 12-13 DPSers, i.e., half the raid), why on earth would you fill the raid entirely with hybrids? There's simply no point - it's not like hybrids bring more buffs in Wrath anyways.

On the contrary, what the 'issue' is, is that you could technically fill up a raid full of 'hybrid' classes, have 12-13 of the spec DPS for Twins, (from your example), and completely sideline classes who don't have a tanking or DPS tree.

I've pugged Naxxramas with raids that have 9 Death Knights in a 25 man, or 4 Paladins in a 10 man. It's POSSIBLE.

The key thing to remember, though, is that just because it's possible doesn't mean you don't want classes that can't tank or can't heal. If I know an amazing mage who puts out amazing DPS and doesn't do stupid shit that wipes the raid, he's going to get preference over a druid who can tank but sucks at DPS. Or a Ret machine who pushes 5k dps but dies in fire half the time.

If having the option to respec to tank or heal, playing sudoku with multiple gear sets/enchant+gem decisions, at the cost of less DPS-oriented speccing options (which is true - compare for example what Surv vs BM vs Marksman brings) is your cup of tea, then by all means roll a 'hybrid' class. Don't do it just because you think it'll benefit raids more.

If you play a hybrid - and are a terrible player - nobody is going to care what utility you bring to the raid.

Yeah, even if you can respec doesn't mean you have the gear to cover that new job. Hybrids can respec on a whim right now and I don't usually see "zomg no hunters QQ".

Ya, my point was simply that once you have enough hybrids to cover all the healing and tanking that will be necessary, there's no advantage to filling your remaining DPS spots with hybrids instead of pure DPSers (you could fill those spots with hybrids - and you will if the best players you have happen to be hybrids - but you don't gain anything from their "hybridness" if you do so). You'll take your best healers and as long as certain minimums are met (i.e., you have enough healers and tanks) you won't care too much what class they are (though you probably want a reasonably balanced raid so you can handle the kiting/spellstealing/mind control/interrupt/stun/etc. gimmick fights).

Because your hybrids outdps the dps-only classes? As things stand right now warlocks only have 1 decent dps tree, affliction, also the one tree that can provide a good raid buff (talented CoE). Demo is so far behind and only gives anything decent past 3200 spellpower and destruction gives nothing and is still behind and as things look, will get even worse with better gear.
So would you bring a demo lock that does around 60% of your mages' dps only to get a 10-20 spellpower buff to your raiders? Or a destruction lock that is on par with Balance druids, on 80% of mage dps? The big unknown atm is debuff slots because a raid can't sustain more than 1 affliction warlock as things stand with debuffs but there are anecdotal reports that a change has been made.
In the end it boils down to warlocks getting nothing compared to TBC. Still stuck to 1 spec, affliction this time, rest are or will get way behind. 10%+ of the dps is very pet related and pets aren't anywhere near resilient and we don't only use 1 GCD to heal the pet up. This time around it doesn't even look like we're top caster dps with that one good spec.

Still things do look nice for now, at least in words. It all boils down to how fast Blizzard reacts to data from the live environement when Naxx raiding starts. If they do what they did to mages in TBC then all the nice words mean jack shit.

This does raise an extra interesting issue I'd not really thought of - with dual-classing may come more of an expectation for hybrids to maintain two sets of raid-quality gear. I'm not certain, though; tank gear is probably adequate DPS gear in most situations, and healing gear is probably adequate DPS gear in most situations (I guess +hit would be the big flaw there), but a druid/paladin tank wouldn't be able to easily transition to healing or vice-versa without having an alternate gear set.

I'm now wondering if I ought to build that factor into our planned Wrath DKP system.

This. I don't see how this thread could possibly be useful, and I'm tossing it.