There are quite a few small things that add up to the UI feel for me. For example
- The mouse dragging feels sluggish and like it lags behind. Also, when you reach the bottom of the screen, you can't go up anymore using the mouse for a good second.
- There doesn't seem to be a way to drag to the left and right, have to use WASD for that.
- Zooming makes the upgrade nodes jump around seemingly at random.
- Feels like lots of sound effects are missing. And the ones that are there don't feel satisfying.
- It's not nice to look at in general. It just seems like it's missing a lot of polish.
I would also suggest having a few nice backgrounds and playing around with the random color on the balloons a bit. A lot of them are very dark. Might want to use a higher minimum brightness value if you're using HSV color space.