What Is a Roguelike?

Jay Barnson has posed this very question over on his Tales of the Rampant Coyote blog, after which he takes a moment to provide his perspective on the subject.

Some folks demand the ascii.

And some folks will deny that a roguelike is actually an RPG. This strikes me as a little silly, as Rogue was exactly an effort to emulate the pen-and-paper RPG experience on computer. But that's another story. But hey, that's why we're opening up the discussion here.

Ultimately, we're talking games inspired / derived from the original Rogue. There should be some kind of family resemblance in there somewhere. What does that mean to you?

There's the obvious children of Rogue - Nethack being the crown prince of the lot, with siblings Angbad and Adom also mining the same vein.

But then you have the incredibly complex monstrosity (and I mean that in a good way) that is Dwarf Fortress. It's about as roguelike in its presentation as one could ask... yet it shares little in common with the dungeon-delving RPG gameplay of its predecessors. That would kinda violate my loose definition of a roguelike above, yet I don't have the guts to call it out and declare it as not being a roguelike. I'd feel silly, and we can't have that.
The only ASCII roguelikes I remember enjoying were DND and Telengard (yeah, that was a long time ago...). I've never tried Dwarf Fortress, but it sounds like I probably should when I get around to it.