Oh, I am all sorts of particular in my design:
-Every level has a custom palette
-Grassland palettes, however, are always the same as normal except in pastels
-Every level has 2-3 secret rooms that are completely unrelated in gimmick and graphics style (and usually song, too) per level.
-Every level becomes 10 times longer if you want to collect all of the Dragon Coins.
-Every level has a reward (be it a secret exit, extra lives, or a shortcut) for collecting all the Dragon Coins.
And in addition to those quirks, I have a tendency to spam:
-Guiding coin lines
-A hierarchal layer structure in my levels
-Arrow signs
-Teleport blocks (for walking/falling/flying up off the screen to another room) cuz pipes and doors bore me
I could go on for days, but I'll stop now :3
Visit me at youtube.com/user/uberniftyman for LP's and videos of my hacks!