Some suggestions: 1) Make the levels more obvious. It's not clear why players are stronger than others for some time. It would also be nice to see how far I need to go to level up more clearly.
2) ADD CHAT! I can't emphasize this enough. We need to be able to yell at each other and complain and work together.
3) Add the ability to switch classes more often.
4) Make the differences between bots and humans more obvious. I want to immediately know when I am fighting a person. A different color name, make them glowing, something.
5) Show the leaderboard after you finish. I want to be the best player today, or the last hour, or the last week or whatever.
6) Don't deduct any stats when I level up. Sometimes when you level up you are suddenly slower. I shouldn't get punished for leveling up.
7) Show my kills against different players. Let me develop a grudge. It makes it fun.
8) Show a message when a player kills a player! Let us know something cool happened.