Super cute graphics and story! I love the idea of Mr. Licorice as a humbug! Production values are top notch too, everything feels super smooth and polished!
I also struggled a little with the WASD, space controls (funny because you are right, that is exactly the combination used by many a FPS and I've never had a problem with those).
But my main issue was aiming the warp gun with the mouse while jumping around. I found it very difficult to accurately fire the warp gun because every time you jump the whole screen scrolls causing everything to shift around. I found I really had to think ahead about where stuff was going to be on screen and plot my shots accordingly. Well, probably that is the whole point, but my pea brain just couldn't keep up!
Maybe a little 'auto-aim' assistance would help. Or perhaps a slower difficulty ramp with the warp jumps. It does jump pretty quickly from basic warp jumps to reach a higher point to chained warp jumps over a spike pit... As a related aside, I actually found the jumps required to find the hidden treats quite a bit easier than the jumps for the main path.
Also, the range of the warp gun was quite short. Does it get upgraded later? Otherwise, you might consider extending it. As it is, the short range just makes hitting your targets that much harder.
All that said, the game is a lot of fun and I'll keep at it to see if I can master that warp gun yet. :)