Climbing ladders as the second player is too difficult and there is no indication that getting hit by a barrel is bad until after it occurs. I can't really call the barrel thing an "issue" though, because you can learn about the stun fairly early in a game and with no need for any real learning curve the explanation that barrels stun doesn't matter. Helheim, it is a good example of learning through gameplay. But the ladder accessibility is just bizarre and seems to only function half the time while remaining motionless.
I haven't studied the game enough to really consider balance. The asymmetrical style of play is difficult to challenge on the premises of balance because I can't really find a feasible opponent equal to my own level at home.
For a ludum dare this game is well designed. You have to give a good deal of credit to someone for programming something (that works) within 48 hours.
Personally, not my type of game. I think the designs for the mock Marios and Luigis rather ugly along with the background, and comparable to other flashes it doesn't excel far with only one map and far too many traits in common with the classic that it doesn't appeal much to me.
That said, I still respect this title and although I don't like the game because of my own pickiness, it is something easily appealing to those who enjoy good nostalgia with a twist to a legendary classic. And I think the best thing is that it fills the shoes of Ludum Dare 25 with ease and excellent execution.