If a critter gets knocked unconcious it loses a level, this makes it remarkably difficult to keep it alive. I've got a level 13 whale while everything else is level 22 (and two are over level 30).
I can't keep up with their combined hit point total. I was saving up for Anti-Missile Lasers so I could deal with the incoming missile threat and in the process was annialated due to an inability to replace ion cannons as fast as they were destroyed.
Economic center is not worth it. Ever. I don't care how often the money comes in, the base center takes 200 ticks of that at a population of a billion (hard to maintain) to *break even.*
I dislike the buggy collision detection, there have been numberous times where I've jumped into a block only to get shunted around it. Lost first place to the newbie map because of that. Took me three jumps.
Upgrading to get Ice and Poison towers half-carries between games. The research building thinks I've paid the money, so it won't let me upgrade, but I can't build the building.
OK, the orange was challenging, but only because it was mostly the same color. Had to form a frame of reference (matching pieces) before I could complete the picture.
 "Brute Wars"){kind=icon}
 "Cog Factory"){kind=icon}
 "Virus"){kind=icon}
 "PlanetDefender"){kind=icon}
 "Platform Racing 2"){kind=icon}
 "Color Jigsaw"){kind=icon}
 "Sinta:Escape from Ixerron Keep"){kind=icon}
 "Heist II - Greed is Hell"){kind=icon}
 "Castle Crashing \"The Beard\""){kind=icon}
 "Billiards"){kind=icon}
 "Hexxagon"){kind=icon}
 "IceScape 2"){kind=icon}
 "Massive Space Tower Defense"){kind=icon}
 "SeaScape"){kind=icon}
 "TiBiA FlaSh"){kind=icon}
 "Hybra tower defence"){kind=icon}
 "JigCircle"){kind=icon}