It's a great game, but there are a few flaws.
The crimson, great idea, but it's very nature makes it worthless in the game. It either needs to be made more powerful or have it's cost reduced. As it stands it's not worth employing early game due to it's prohibitive cost, and not worth while late game because it will almost never get a kill (and thus never increase in power beyond leveling). It also loses some of it's worth around lvl 10 when it can't one shot kill anything anymore.
Lagging, I don't know how to fix this issue because the issue has to due with the number of blood splats on the map, combined with the number of shots fired/sec with the number of guns. Once you get to about lvl 25-30 it starts to slow down and ends up grinding to a near halt once you get into the 80s or above.
Leveling glitches, when the guns level while there are still enemies, way to often they won't shoot. It only happens for one shot, but always immediately after a level up.
However the positives more than outweigh the negatives on this game, the randomly generated maps are fantastic, the unlocking of the weapons, the various kinds of enemies (I hate those healers, especially when they're grouped up, which makes it sooooooo much more satisfying to murder the crap outta them.), the wide variety of guns and the ways they can be used to augment each other, the various strategies that can be implemented with the random maps.
All in all a great game.