Sprites:
Newbie Custom Boss (The N00b ultimate Boss makes it obsolete, kind of)
Reactor Core bosses (because the above is used a lot)
Thwomp Boss (even the Japanese are overusing it now!)
The standard Custom Boss List (Mouser, Lakitu, Fishin' Boo, Ground Pound thing, Big Boo, Sledge Bro EX, Classic Bowser). It's what every single mediocre hack will use for the boss list.
Cut Scenes
Thwomps in various directions and with various edits.
Sky Pop. It's so clunky and slow, and every featured hack seems to use it.
Graphics:
Any Super Mario Bros 3 Styled.
Donkey Kong Country and Yoshi's island graphics
Redrawn graphics
Skeleton graphics (they look horrible)
Wario replacement graphics and enemy replacements (they look bad as well, and are often used by Wario themed hacks merely because they exist)
Music:
Stickerbrush Symphony (talk about killing a good thing!)
Krook's March
Yoshi's Island Castle
Anything SMB 3
Other Stuff:
Default Koopalings. Yes, they're default, I know. But the fact I now near enough hate the characters pretty much shows how much they're used.
Deep Plot Lines (I don't want plot in Mario. The market doesn't want plot in Mario. People don't generally want video game cinematics period, according to someone I read called Malstrom)
Brambles
Non linear puzzle design (that ain't Mario!)
Overly boring level design wtih ASM replacing the quality.
Muncher Runs
Super Mario Bros 1 first level remakes
Sonic the Hedgehog as a Mario character (Sonic lives on Earth/Mobius/somewhere I don't know, Mario lives in Mushroom World. They're not from the same series!)
Marx as a custom boss (it's everyone's first idea for some reason)
Whispy Woods as a custom boss (ditto)
Luigi as evil (too many Japanese hacks. Luigi does not want to kill Mario people!)
Underwater Scrolling Levels (oh boy, everyone's worst nightmares in the same level!)
Yoshi player characters
Self insert maker villains
Custom Koopaling bosses (again, if you know ASM don't remake the Koopalings!)
Broken overworlds
For gaming news and Wario discussions, check out
Gaming Reinvented and
Wario Forums respectively.
As for Mario's Nightmare Quest? Well, it's currently on Fusion Gameworks, ROM Hacking.net or the GCN at the moment.