Incredibly, it seems that the funnest level out there so far is "Experimental: Flinging", with 4 or more players on the 2P map. It's quick, mindless and spectacular. My goal is to make a level that's as fun as this one.
Found another problem today: the nuke. With knockback explosions (heck, even without them) this is extremely disruptive. One player can destroy the whole contest very easily. I suggest the nuke be disabled until one player has an unassailable lead. For situations where this results in an unresolvable stalemate (eg. only blockers left), let the players launch the nuke by all pressing the button at once.
And another random thought: how about hotkeys to move the view to your entrance or exit?