You earn way too little XP on challenges. If I have to level up just to get the first tier reward on the shield challenge, I should be able to at least level up once after a couple games.
I actually have a suggestion rather than a complaint. How about before a match starts, a last-minute prep period is given, allowing you to set priority targets? And maybe a setting dictating how close your bots should get to their target?
@UFK: It took me forever to find it, too. It's that tiny money sign in the bottom left corner of the screen. It doesn't seem like much, but when you hover over it you find that you can activate it to sell your towers.
What would be fantastic would be an option after attaining a five-star hotel that would let you demo the whole building. That'll teach those idiots badgering for a twenty-seventh elevator shaft.
Fever mode is given when you hit the maximum combo, which is determined by it's respective upgrade. But, there's no badge or award involving a maximum combo, so go ahead and buy the upgrade, as I'm fairly certain the combo affects how much money you receive.
 "Papa's Freezeria"){kind=icon}
 "Berzerk Ball 2"){kind=icon}
 "Bot Arena 3"){kind=icon}
 "Effing Worms - Xmas"){kind=icon}
 "Infectonator 2"){kind=icon}
 "Bunny Flags"){kind=icon}
 "Theme Hotel"){kind=icon}
 "Epic Combo!"){kind=icon}
 "Help! It's the Unfinished Shadow Game"){kind=icon}
 "QWOP"){kind=icon}
 "Shopping Cart Hero 2"){kind=icon}
 "MAD: Mutually Assured Destruction"){kind=icon}
 "Mardi Gras Mayhem"){kind=icon}
 "IndestructoTank! AE"){kind=icon}
 "Mushroom Madness 3"){kind=icon}
 "Scriball"){kind=icon}
 "Color Link-a-Pix Light Vol 1"){kind=icon}
 "Pandemic 2"){kind=icon}