for osu!mania:
no pro osu!mania player plays with a ddr based (or anything similar) arrows as the notes nor use a clear playfield.
grid colors should be alternating, mania-hint should be perfectly aligned to the judgment line, and note colors should be contrasting.
alternating grid colors are better than alternating grid transparencies with the same color.
some people prefer copying the note color setup from whatever similar game they play to make it more familiar.
border between stage sides and playfield should be noticeable.
if you plan to not use column lines, make sure to at least put line(s) on the center column(s). those center lines will help you a lot, especially in 6K, 7K or 8K.
don't use overrated stage lightings. (the lighting that pops up behind the grids when you hit notes)
try not to use lighting N's and L's that are too flashy, people focus their eyes on that point when they want a high accuracy (although I use a flashy one since I look higher on the screen while playing)
"don't block the driveway", hitbursts will always be there, so never make your hitbursts too high. smaller or thinner is better for some people. just don't forget to make 300 different from 300!.
use a larger miss hitbursts than the other hitbursts.
i prefer using note holds with a vertical gradient, it tells me how long a slider is and tells me when it's about to end.
make your stage right and stage left as fancy as possible, no one looks there while playing.
try not to make the hp bar too distracting.
taiko:
i don't really play taiko, but I think it would be better if the stage right contrasts the note colors (blue and red-ish)
don't use a distracting pippidon animation
colors should be dominant on the outer part of the circles
don't use a stage right glow that is too bright/glowing
make sure the hitbursts don't block incoming circles
make sure that the border between stage left and stage right is noticeable
ctb:
i also don't play ctb but...
use drop shadows for the fruits
try to make the fruits look as closely as possible to each other
make use of the fruit overlays to add lighting effects and shadows to the fruits to make the fruits more visible
try to make the font simpler, thinner, and smaller
meh can't think of any more now >.<