Okay, I've played a little and I think I'm ready to give some feedback. I'll definitely be adding more as I play more, but I thought I'd give an absolute first impression.

1. It seems sort of unfair to judge the game mechanic as it is now. The fact that most of the activated abilities can't be used yet sort of throws the system for a loop. The opponent's activated ability is incredible, and it doesn't ever get the chance to use it.
2. The animations are great, but a couple -- especially Supernova! -- could do with some speeding up.
3. The field of view is suffocatingly narrow, but I assume that too is just because you're using a beta engine.
4. Did you mean for the Arcane Burst effect to be "up to 4" or "exactly 4?" As it is, it is exactly 4 and I'm not sure that's necessarily fair.
5. It'd be nice for the Change View and Done buttons to be a bit larger -- or at least colored differently from the background so they stand out a little more.
6. Effects seem to stack for allies, but not for heroes. I know Kyle said that effects won't stack at all, but I actually kind of like the idea of being able to use a "burn" effect for heroes. I'm not sure how to balance it all on to one card effect though, since burning a hero is usually a lot more powerful than burning an ally...
7. An undo button would be VERY, VERY helpful. I can't tell you how many times I've sacrificed a newly-drawn card by mistake already.
8. There seem to be a dearth of cards with similar effects: Freezing and Webbing are the same effect; Engulfing Flames and Poison Gas are the same card with different art. I've never read anything about whether there is a cap to the number of identical cards you can have in a deck, but this is a way around that rule.

More to come!