This is great! Funnily, I was talking today about how I've only played the second Risk of Rain and not the original (or new) 2D version. Now I feel I have. :D
Power ups stack pretty quickly and I was soon wrecking everything without problem, even though the difficulty timer hit the max level. Which is fine by me, I enjoy being overpowered in games!
It was a nice touch to have the previous level bosses spawn in during the next boss fight, it helps give the player the feeling of progression since the first time facing them.
I see the jump bug has already been detailed so I won't mention it further. Aside from that, it was pretty difficult for me to go down ladders which standing above them, the precision needed to use them was high. But there were always convenient cliffs nearby to jump off of instead. :)
I also appreciate the gamepad support!