I voted for Simplicity, Source Code and (because I didn't think it through) Clones. (Should have gone Pixel Art.)
Simplicity is good because it means there's no pressure on people to make something fantastic and because (let's be honest) something complicated isn't likely to get played much on a tiny screen and will always be limited by hardware restrictions.
Generally I'm not a major proponent of open source (certainly not Richard Stallman level) but because Arduboy is meant to be educational I think it's important that people make the source code of their games available for others to learn from.
Clones of classic games are nice because nostalgia and making things easy, but also because most people know how they work and what the rules are so they're good to try to copy and to learn from.
Pixel art because dem pixels. (P.S. really want a 4 colour screen. Black, White & 2 Greys)