There are many less limits than you think. The main limits are browser problems, and poor HTML5 performance on mobile devices.
The only engine limits I can think of are
-no 3D support, as Rory said
-no multiplayer, again, said by rory,
-no vector manipulation or vector graphics (custom plugins might cover this)
If you have any specific examples, we could probably tell you if it's possible/feasible within C2. 99% of things are.