There loads of disagreements on what hack and slash is supposed to be. There is no definition of the term to be found anywhere. From wikitalk:
"This article is a bit of an WP:OR magnet, since it's a popular term without a clear definition. It's also a label people like to throw around to kind of insult games (oh, it's just "hack and slash")."
I think its a subgenre if anything. So you get stuff like:
HnSRPG (Diablo, Baldur's Gate: Dark Alliance, Dungeon Siege, et al)
Crowd HnS (Dynasty Warriors, Ninety Nine Nights, et al)
The Third person action games are an offshoot from platformers, so I don't count them at all. Ninja Gaiden is simply the 3d incarnation of the NES and SNES games. But that's just me, I could just as easily call them HnS Action games.