Add whatever they call them charges for Unholy DKs to empower their ghoul. And hunters have a stacking thing going on too.
Lots of classes are getting abilities/talents/resources that require stacking and unleashing in order to allow for more variation in play than a 1, 2, 3 rotation. The goal is for you to be making active decisions in order to increase DPS rather than it being more a function of latency and gear. More reactive, if you will.
I don’t think any of them are as severe as the combo points for rogue/feral, however (where if you don’t know how to use them you can’t do any sort of reasonable DPS). Just a way for you to vary your play and perhaps a larger gap between “good” and “bad” DPS for a given class with the same gear set.
The example for the DK is that as you (or it?) crit you build up charges and once they stack to 3 (5?) you can empower your ghoul into a monstrosity (think Deformed Fanatic in Lady Deathwhisper in ICC).
NOTE: Posted here cause Skammer recommended it.