I know of a few of the interesting buffs to spellsteal (core hound transformation from the Shadow Lab) and apparently the greenwarders in Botanica can give a huge +dmg buff from spellsteal or mind control.

Anyone got a list of the useful buffs that can be obtained from Mind Control or Spellsteal? Also, which mobs are useful to be mind controlled when clearing trash?

The Astromages in Mechanaar have that fire shield debuff, 600 a tick pre spellpower and you look like you have Geddons bomb.

Zealots in Shadow Labs - around 20% (I think) they turn into a mini-core hound which silences you but gives you an attack buff that lowers attack speed.

Botanica - you can get a +295 dmg buff from some of the casters

Shattered Halls - +105 Shadow Resist (lock couldn't figure out why the mobs kept breaking seduce)

Weird / Bugged ones (or ones I forget where they come from):

The firewing mobs in Terrokar - you can steal what looks like PW:S but doesn't give you a debuff/buff, just puts the bubble around your toon.

Theres another +150 dmg buff in one of the instances around Shadow Labs (the one with the bird people)

For instances that are a bit lower (dunno if you're even interested in these!), darkcasters in Mana Tombs absolutely plow their buddies with their massive manaburns and SW:P spells.

Some of the other mobs in Sethekk halls aren't bad either for MCing.

MCing Gene Splicers in Botanica and using Death & Decay to kill each pull in roughly 15 seconds is something everyone must do.

Are all the abilities mentioned here removable with Purge? I know that the Shadow Lab Zealot doggy-buff can be purged, it's the only one I tried though. All the resist buffs are things I never would have realized were on a mob.

Confirmed. It works but the view is horrible and he runs slow =\

It's pretty fun though regardless of the limitations. They should put the camera view above his head instead of between the legs.

Boss-wise, you can spellsteal:

Temporus' Hasten (totally useless, but 100% movement speed on a mage *is* hilarious, and if you don't have a shaman or priest to purge/dispel, it's an alternate method),

Julianne's Devotion (50 or 100%, forget which, casting speed increase for 10 seconds)

Romulo's Daring (useless, but again alternative to purge/dispel)

In Arcatraz, the Wavecaster ethereals also have a stealable cast-speed buff.

The air elemental mini-boss in the Arcatraz final event casts a Deaden Magic effect (50% less spell damage taken). Stealing it provides a nice two-for-one effect that I enjoy a little too much.

What about Demons? Anyone care to mention any demons with nice things to use?

I tried this on my 61 priest, but it seemed like it would break extremely quickly. Maybe I need to level engineering!

To expand upon this, the fire shield receives full bonus from spell damage, so our mage was doing 400 DPS to three mobs at once since it was ticking for 1200 damage every 3 seconds. Needless to say, he learned to not stand next to the mobs that weren't being assisted down.

The shadow priests in shadow labs are good fun too. Their mindflay does a 1000 dps and the dot does about 300. They are quite viable to help offtank those large pulls.

The green satyr mobs with scythes in botanica have a 35%(!) stamina reduction curse debuff, melee for 1100 and cleave 3(!) additional targets for 1400. Why the fuck can't they give that to warriors already.

While not in an instance, the Maidens of Pain in the cave next to Honor Hold have a 30% attack speed buff for 5 minutes on a 1 minute cooldown. Castable "on allies"

In Karazhan, I can steal the buff the Ethereal Spell Filchers get (Steal Magic). +8% damage plus it makes you grow a little and turn transluscent. With ten buffs on me my gnome was taller than a human. :)

Sadly, I could not steal the Shadow Magic buff from the first Mechanar mini-boss, despite it being a magic buff.

For Warlocks, I'm sure most of them know how powerful enslaving the demons are in Mechanar (to clear the netherbomb trash packs) and Botaica (the Satyrs are insanely good).