I've seen the term "Metroidvania" mentioned quite a few times now. I'm guessing it is just an empty buzzword, but nevertheless... How would you define it? Please bear in mind that I have only seen a little of Metroid and Castlevania back in the day. Oh and I know that a later Metroid is first person, but that's about it.
blobber level design + NES action platformer gameplay + automap - difficulty of either = metroidvania
EDIT: 2D sidescrolling viewpoint