Swinging the axe is REALLY FUN.
The upgrade tree is difficult to navigate. I would appreciate it if icons for upgrades I can't purchase are greyed out so I can more easily see what I can purchase.
The loop was basically: Clear map -> Upgrade -> Repeat.
A little tedious after a while moving my mouse hand around so much, maybe some upgradable automation could alleviate that (if it's not in already, again had a hard time with the skill tree).
Try making it so tooltips stay within the game window. Often times I had to reposition the skill tree to read what a skill did.