Hahhahaa! "It's an fps/rpg-hybrid! No! It's first person action rpg!" That's gotta be the one of the oldest fights on the face of the Internet. I didn't like it in TTLG, I absolutely hated it in ISA boards and a year ago it was just stupid here.
On Thief, I can understand where MN is coming from with his argument, even if it's a flawed one. He probably thinks that Thief differs enough from regular fps' with it's heavy focus on story and narrative and the relative lack of fireworks to not condone with the definitives of the fps genre.
The sad thing is, a few years back I would have strongly agreed with him. Yes, it would be wrong to clump Thief together with such games as Skynet or Quake, or even the recent Far Cry, but the fps genre is so broad that it can easily include genre-bending games too. Sneaking and a store model don't make an rpg. Thief has absolutely nothing to do with them. Nothing.