Isn't this whole topic fairly pointless since we all know this stat is very benifical offensively (threat) and defensively (fewer parry strings)... but more than anything, there just isn't enough ways to "stack" this stat to make it really anything more than a frill.

If we start seeing enchants and gems with Expertise then I think more math will be needed. For now, just keep stacking stamina and enjoy the few pieces of +expertise gear that you can afford... it's just gravy for now.

Yes and no. Specifically lets take a look at [Brooch of Deftness] and a similar ILevel [Frayed Tether of the Drowned].

Assuming you can always hit crit-cap, many love the Brooch for trash, but would still use the Tether for for boss tanking, right? Or does the parry-mitigation combined with the threat generation make the Brooch better?

Then again most may still just use the [Barbed Choker of Discipline] before upgrading to [The Darkener's Grasp]

Not exactly pointless. Consider my situation:

I have a crappy tanking belt (from Heroic Ramparts) (Lion's Heart Girdle - Items - World of Warcraft)
I have mediocre bracers (the ones from Attunesmen) (Vambraces of Courage - Items - World of Warcraft)

Is it worth it for me to buy Bracers of the Ancient Phalanx (Bracers of the Ancient Phalanx - Items - World of Warcraft) to upgrade my tanking set, or would the belt, Irontusk Girdle (Iron-tusk Girdle - Items - World of Warcraft) be a better upgrade for overall tanking.

Of course, my Lion's Heart Girdle is so crappy that the Iron-Tusk is a far better upgrade, but you see my point. It's not JUST an academic issue.

edit: Sordee beat me to it, and gave a better example than I.

While at first I was gonna say "expertiese would be a better kind of avoidance becuase it's a "parry-gib"-avoidance" I thought again and figured that dodge/defense/etc are also "parry-gib"-avoidance as the attacks actually need to land in order to parry gib you... So if you're looking at the total chance for a "parry-gib" to happen there would be a breakpoint for a given gear setup where X expertiese is worth Y avoidance. However if you look at "chance for N hits to happen under X seconds" this ratio would be very different and dependant on X and N, as if for example X<boss weapon speed and N=2 then reducing parry VS increasing avoidance will have a different relation than if N and X were very large numbers.
Basically the defensive capability of expertise really depends on what actually kills your tank (doube block in <1s = dead tank? or does he also need to crush after that? etc). However given what you actually want to avoid, expertise and avoidance *are* interchangeable when it comes to avoiding that.

The puchline would be though that since boss parry change is just way too high to reduce to 0 or anywhere near it, you will still need the stamina/armor to handle whatever parry gib type you chose you want to reduce the chance of happening, thus expertise, like avoidance, will never be able to replace stamina/mitigation for survivability, assuming a hard hitting boss that can kill the tank in less than "several hits" (~6 or something like that was theorycrafted on another thread).

On a related note:

After causing a couple of wipes on Al'ar due to resisted taunts after Melt Armor, does anyone have any data whether Expertise will also decrease the taunt resist rate (ie. effectively can taunt be dodged/parried in the new system?).

Just wondering if I can swap in some expertise gear, or only +hit gear is gonna be useful to reduce taunt resist rate?

It's an academic question at the moment, because the stat has only been just released and for tanks generally appears only on gear that was just released, which is all T5-ish level. It's going to become a much more realistic concern when it enters mainstream itemization, which it will at the release of Sunwell Plateau gear, and again in WLK, and even more so if blizzard decides to do away with crushing blows (as a boss mechanic). We very well could end up in the future where it is possible to expertise-cap, and possibly even juggle around pieces of equiment to stay around the cap, and I think it's worth it to get a head-start on what sort of stamina/mitigation/avoidance it's worth to sacrifice to get to that point. There's certainly no harm in seeing what sort of effect expertise will have eventually even if that won't be for a while.

Some mobs cast while attacking, though few. One in particular is Gaithos for the Illidari Council. If a boss can parry while casting, unless that boss is attacking while casting, then a parry is meaningless, it does not add haste to the next swing because the next swing is going to be the first, nothing to haste it towards. Can't make a first attack faster if it cannot attack while casting.

Expertise might be a tad overrated as it matters only in fights where the tank is undergeared or has a regular insta-gib component to it. On regular and farm content expertise only increases threat.

Expertise as a mitigation stat is probably overrated. It is, however, a very good threat stat.

This is simple rewording to what I said, albeit probably better worded as well. Also, since last post, I can claim that parries can indeed occur during casts. Balinda in AV parried an attack twice in a row this past weekend for me.

While the threat gain is undeniable, let's not be so quick to write it off as having valid mitigation value as well. Of all roles in a raid, tanking is arguably the most min/max oriented. What sort of difference would it make to have 0 parries during a lengthly encounter? Keep in mind reducing the parry chance to 0 also means there can be no 'unlucky parry streaks' taking place either.
If I'm a tank looking to improve mitigation or avoidance, of course I'll look for dodge and armor count. But knowing there's a stat that will increase both my threat generation as well as help out on mitigation/avoidance is refreshing.

Pets, other players, snake trap snakes, and the tank's parries all result in hasted hits on the tank.
Pets seem to be good about attacking from behind. For melee you just have to train them.
There are times that the parries can't be avoided because when a boss turns to do a random target ability on someone in the raid he can parry melee who are correctly standing behind him. This is probably what you are seeing on morogrim as he turns to send people to watery graves.