I miss the ability to upgrade stats in general, the minigun slows you down so when a purple zombie is in the game and next to you, you just die, you cannot jump (bah I wanna jump in a group with a super machete and whack em!)
I found the game pretty easy (even on hard), didn't need the pause button much except for changing equipment or checking gems. Levels on hero and eq on hero helps in start of level :-)
Yep, no persistant upgrades. Also would be nice if you don't spend your energy you get some kind of 'interest rate' on it, so it's a challenge to get faster upgrades in the end if you spend carefully in the start
Still don't have any use for the third upgrade materials, beside selling it for profit. Threaded leather etc... anyone has found combinations with that stuff?