(I should preface this by saying I've been known to respec three times in a day, hence the questions for so many specs)

1) As ret, I've been focusing pretty much on warrior gear (str/crit/hit/AP) ... what I'm wondering is how much benefit does +dmg give me? I've been treating it as 1 +dmg equivalent to 1 AP .. however if I do it like that, most of the time I land up wearing pure warrior gear. Is my estimate correct, or am I undervaluing +dmg? (For reference, I tend to open with JotC, then SoC, CS on every cooldown, JoC/refresh SoC on cooldown as well. Current unbuffed stats in ret dps gear: 572 str, 1502 AP, 20.21% crit, 0 +dmg, 50 hit rating)

2) As a holy/prot spec (41/20 or 41/15/5), I'm wondering if there's an effective way to do single-target damage? stack +dmg as much as possible and just JoR/shock on cooldown, keeping up JoW for mana? +dmg is obviously a primary stat here, are there any others I should look out for on holydin gear?

3) As a pure healer (which I tend to be in some heroic groups, as well as in Karazhan), how important is mp5 for regen? My logic used to be stack +healing as far as possible and use consumables for regen as needed, but I find myself scraping the bottom of 8k mana more often than I'd like ... if I'm at +1400 healing but only 62 mp5 (casting, 200 mp5 oFSR), would it be worthwhile sacrificing some +heal for +mp5? Assuming I added 10 mp5, that's 120 mana/minute, which is 600 mana in a five-minute fight ... doesn't seem overly significant.

4) I'm seeing quite a few items pop up with +crit in place of regen (T4 gloves, boots of valiance off Moroes, etc) - am I right in thinking that even for a holy spec, +crit simply isn't that effective for flash spam?

Just as an aside, this depends on your weapon speed. The faster your weapon, the more valuable spellpower is; conversely slower weapons benefit more from attack power. At 3.0 speed, a point of spellpower is almost exactly equal to two points of attack power.

I like crit rating a lot as a healing paladin. MP5 is much more consistent, but crit gives you stronger heals as well as more efficiency. YMMV.

Just a quick note, according to Another Thread, SoCom has a 20% coefficient (and the Judgment a 42.9%). CS gets 40% of spell power (from the spell description).

Assuming a 3.8 speed weapon (I'm laboring under the impression that both CS and SoCom procs are unnormalized), to add 1 damage to CS, it requires about 3.68 AP or 2.5 spell damage. For a SoCom proc it requires about 5.26 AP or 5 spell damage, but that 5 spell damage also adds about 2 damage to the judgment, whereas the AP adds nothing.

So, for CS and SoCom, spell damage itemization adds more damage per point, but of course this is balanced against the fact that you get white damage from AP... of course that's also subject to mitigation, and doesn't concern additional sources of Holy damage (mainly consecration, which gains 1 DPS for roughly 8 spell damage).

I stack +healing & crit especially for heal botting. There are easy ways to pick up mp5 (flask of mighty restoration, mageblood, etc.) However, I'd advise you to get an Alchemist's Stone. Using super mana potions it is the equivalent of adding 30-50 mp5 (in a fight where you're required to pots every cooldown). With fel mana potions it is the equivalent of 54 mp5, not too shabby for a trinket that has +15 to all stats on top of it.