The screen is so freaking tiny. You can only see like 10 feet in front of your face. Which couples with the fact that they seem to very frequently spawn in your face.
The physics make sense in an odd way, despite earlier comments. However, this game is based around a poor control scheme. The "new" method of movement is certainly new and uses physics, but it is not fun.
I found the original game to be much more imaginative and fun. This one feels too much like shooting in the dark and hoping you hit the enemy more than he hits you, rather than dodging or any of that sort.
I don't like how each unit has so many options. Before it was as simple as, "bam, fire mage", but now I have to remember how many debuffers, slowers, etc. I have and if I have too many and if I should specialize and... Yeah. That's too much micro.
Dying in midair while executing a killer combo is stupid. Additionally, the build up before one can execute the killer combos after dying is also just as stupid. At least let us restart at the level we died.
The lag on the cursor is horrible. Gotta have a lot of precision when it gets tracked at like a 20-50 ms delay compared to outside of the flash window.