I hope the MOAR DOTS achievement can only be achieved by not using more than 12 different kinds of DoTs for 10 man, maybe 30 for 25 man. Make it feel like serious old school Onyxia.

The true achievement is thinking of 30 different kinds of dots.

Let's think about this...

DK's: 2-3 DoTs depending on spec.
Druids: Ferals have 2-3, Balance has 2.
Hunters: 2-3 depending on spec.
Mages: Very brief, but 1 or 2 again depending on spec.
Paladins: Tricky. Seal of Vengeance/Corruption may count, but otherwise not much.
Priests: 2-3 if Shadow.
Rogues: Deadly Poison, plus a bleed or so.
Shaman: Flame Shock, but depends on the Shaman's preference
Warlocks: The big guys, at least 5 for Affliction, 2 or 3 for the others.
Warriors: Rend.

Ok, so let's assume you get one of each class as a DPS role in your raid. Assuming maximum DoT potential (neither of these conditions is likely to be met in your average group), that is 25 DoTs by my count. Even if you set the group up to get the most dots you won't surpass your limit of 30. Because of this, I believe an alternate achievement would be better.

"OK STOP DOTS (10 player): In Onyxia's Lair (10 player), kill Onyxia with a damage over time effect dealing the killing blow."

Unholy Death Nights have 4 now with the new Unholy Blight.
Priests have four because of Holy Fire.
Presumably everybody could grab themselves a weapon which applied a dot effect.

... Why am I still talking about how to accomplish a made up achievement ^^