Like everyone said, the long load times were terrible. Especially in Insane when you can die in an instant of carelessness and have to endure a ~45 second load time.
Other than that my other biggest gripe is the difficulty. This game is NOT hard. It's unfair. There's a big difference. On Insane there are a few problem areas that are an exercise in frustration not because they're difficult, but because they are stupidly cheap.
There are a few minor gripes that aren't game hindering but a bit disappointing like the texture quality/pop, terrible animations/game models. Some very dated mechanics such as not being able to hold more than two guns at a time.
But all in all it was mildly entertaining. Not mind blowing by any means, no. But it reminded me of DN3D somewhat. Not quite as bad as people have made it out to be, but not great by any means. I'm glad I only paid $3 for it, though.