You missed this Reduced Quality Thread
but it's ok
In Picture Quality if you are using the same application like the Android Gallery(cooliris) in 2.1 then on custom rom(2.2./2.3) it will be the same, because that Gallery downsizes the Images, so quality is reduced.
in Stock 2.1 Album, don't have that problem, or in custom roms, try to install Quick Pic. Quality will still be the same.
but of course can't compare Picture Quality from a computer to our X8(limited to 65k colors)
About Video's i never noticed the quality in 2.1 or in custom roms, I use Mobo Player, which can play all kinds of formats too. although i still use 720x480 mp4 for my video's if i decode them.
about Gaming 2D or 3D, yes this is very noticeable and really depends on what ROM you are using. I use FroyoBread during the early versions 2D/3D is not yet fixed so quality is reduced and can some Gradients/Pixels.
But FroyoBread reached its Final Version and 2D/3D is Great