Once I unlockd the some of the stronger shotguns and heavy weapons, it became really fun. It's easy enough to unlock because quitting mission matches still gets you all the bonus XP, so you can level up really quickly once you get to the end of the missions.
I don't like how you can spawn at directly on top of enemies. Even with the invincibility you get, it usually just leads to a cheap death.
I'd like more game modes. A team elimination, a king of the hill, maybe a tag mode. Being able to customize your team during team modes would be fun too rather than getting random teammates with random equipment. The AI can be derp, but that's generally to be expected. It'd also be nice to be able to customize the different match types you have in, like to add more lives in elimination just in case you got a terrible first spawn, have detch matches go beyond 20 kills, or just have it be for timed.
That said, it's still a fun game, it was frustrating at first but improved once the better guns were unlocked. More customizability in how you play would only make it better.