I think D&D is the RPG most directly threatened by MMOs and will eventually be replaced by them.
Think about what D&D does better than any game: deliver an action RPG experience where you explore the unknown, delve into dungeons, kill things, take their stuff, get more powerful then repeat.
While you *can* do lots of other things in D&D, the experience above is (imo obviously) the aspect of the game that it does better than any other game.
It's also the aspect of the game that can be emulated by MMOs more successfully than almost any other RPG.
In other words, while the dungeon crashing experience can be done very successfully on a computer, other RPGs based on narrative control, or personal interaction (storytelling games) won't be nearly as enjoyable on a computer. This means that D&D is threatened by MMOs more than other RPGs.
To a lesser extent I think any game focused on combat is vulnerable to this. Look at City of Heroes, a game which captures many of the charms of Hero/Champions and other superhero games.
This leaves D&D as a social gathering, which I think can sustain it for awhile, but I think technology will eventually allow MMOs to capture even this social aspect of the game.
None of this is groundbreaking or original, just me compiling a bunch of thoughts I've had, inspired by various sources.