-Every detail, no matter how small, is significant
-Puzzles relieve tension. Place them between areas of high action to calm the player down before smacking back in the center of another fight.
-Model > texture > brush (That is, if you can make things look 3d using models and textures (Like the original game did), you'll probably never crack the 400 RS mark!)
-SOUND, even if its a background mp3, makes any level better. It adds numerous themes, and atmosphere.
-Textures that are 256 x 256 are always better than smaller textures. You can always scale down high res pictures to get a very nice look, as well as scale it up if neccessary
Thats all I can think of now... Wait until I sober up and I'll put some more in.