1) Lag, it comes in waves, sometimes the game is great and sometimes you die instantly to all sorts of weapons sometimes before you can react, sometimes after you have put a bunch of rounds into someone and they don't get hurt at all and then proceed to mow you down.
2) Spawns, the spawn is system is broken, have seen way to many people Re-spawning in my deathcams as of late (seems to be worst on Bootleg, Arkaden, Dome and Mission)
3) Boosters/Cheaters etc. don't really see the point at all and don't see how this is fun at all
In terms of #1, or when you're shooting at at a knifer running at you and you empty half a clip into them (most of them hitting) before they suddenly knife you from 5+ feet away. Coupled with one of my reasons, you then see the Killcam of it and, somehow, to the other person, you "apparently" only hit them once or twice, if at all so "apparently" did little to no actual damage to them and they were "actually" right next to you when they knifed you.