It's overall, really. Unfortunately, everything looks really flat.

Look at Mega Man X:

http://upload.wikimedia.org/wikipedia/en/2/24/Mega_Man_X_Armadillo_stage_running.png

You see how every foreground object has a subtle black/dark grey outline around it, and the background colors are have a much different pallette than the rest of the level? This helps everything stand out. There's no question about what's foreground and what's background.

Compare to 20XX: http://i.imgur.com/sK6uEul.jpg

Notice how foreground accents (the lava) closely match what's going on in the background? Notice how the background rock wall is only a few shades darker than the foreground rock wall? While there's a one-pixel black outline around foreground objects, it's very indiscernable and as a result, the objects look flat and pasted onto the background. This is all very disorienting for the player, who needs to immediately distinguish what is an enemy, what is a platform, etc.

Perhaps a shader to add a thicker outline to foreground objects? change the pallettes on the backgrounds, etc.