don't beat yourself up too much for not really having assets. Apparently you can get away with just colored rectangles.
If your engine doesn't have a build in scene manager you need to spend some time to make one. Even if you don't think you need one, trust me you need one.
Make sure that everything is clear and explained in the game itself.
Sound is really important. The lack of sound was one of the biggest complains I got.
Get more people to play test, sooner. Especially as I had a system setup to automatically make and publish release builds whenever I pushed to master. The fact that the player moves too quickly for some people is something that that would have caught.
A bad algorithm is better than nothing. (Looks at the awful way I generate levels and detect collision)