The most annoying thing is when you have only Melee left and you shove the enemy onto the wall and they have Lifesteal, resulting in him lifestealing at like 3x the rate and taking about 5 minutes to kill...
The placement of some coins are unreachable. Having 3 nano bots isn't that much of an improvement compared to 1 Nano bot as they eventually bunch up. Being too big stops you from getting in reach of any of your borders. Sideways indestructable blocks would make dodging the blocks more challenging.