Quantity cannot replace quality
The game is fun, and clearly a lot of work went in to it (if you count the work that went into the first gemcraft. There isn't a big difference except for the traps.)
I like the trap idea, however the basics of any well-thought-out game are lacking, in particular balance.
The game is very challenging, and since it's usually the last few waves that are deadly, repeating the same level over and over quickly gets annoying. I suggest that the special modes unlock sooner, so that leveling up is eased. On the other hand, repeating a level is more fun when you haven't done it for a while... so this isn't necessarily the right solution to fix the learning curve.
The gems are extremely unbalanced:
A poison trap will deal at least twice as much damage as any other trap or tower of the same level.
Slow and mana-steal have their uses.
Armour reduction would be useful if only it worked (probability of reducing armour is too small)
Splash damage simply doesn't work, even when the enemies are piled on top of each other. (which automatically makes chain hit a better choice) You might want to check if it's a bug.
The abilities are also unbalanced. Replenish is mathematically superior to tower builder, armour, forge and flexible pool. The limits based on level fix this to some extent.
The levels are well balanced (although it gets hard very quickly)
Overall, a frustrating but addictive game.