Coelocanth: You have a point in that very few CRPGs have [i]major[i/] plot changes if you play through the game differently. However, one of the big 'replayability' factors is that there are many minor plot points/side quests that are very different depending on how you approach them in-game. As well, some CRPGs have side quests/companions/subplots that don't even appear if you do the story one way instead of another. Couple that with the different flavor afforded by playing different classes/races or even the opposite gender, and I find CRPGs in general have far more replayability than most other genres.
vAddicatedGamer: True. You can be a thieving, sneaky bastard, a charming, persuasive devil, or a sheer brute. The various builds, character development paths (which also corresponds to different equipment), plus the sidequests based on your choices contributes to replayability of CRPGs, at least the better ones.
But then you're still playing an adventure that you already know. Perhaps it's my experience with pen & paper RPGs. If you've played an excellent adventure or compaign there, you don't say: "That was great! Now let's try the same thing with different characters!" You go of and play a completely different adventure.
Take
The Enemy Within, widely considered to be one of the best RPG campaigns ever. I know lots of people who ran that campaign several times, but always as a GM. Playing the same campaign a second time as a player would seem rather pointless, not to mention unfair, because you already know what's going to happen.
Compare this to strategy games, like Advanced Squad Leader. Lots of people replay the same scenario several times, even against the same opponent. It's a pure test of skill and a learning experience, rather than experiencing a new story.
As such, my feeling is that strategy games (especially multiplayer ones) have an enormous amount of replay value. For CRPGs, after the initial play, any replay will naturally suffer because you already know the main plot. It is now merely a test of skill -- can you do it at higher difficulty? with different stats and abilities? -- and it has that completist/collectors thing that works with single player adventure games, but not with multiplayer roleplaying.
I guess what I'm trying to say is that to me, it feels like any replay value in CRPGs exists only if you ignore the roleplay value. It's a game that needs to be beaten or a gameplay world that needs to be explored, rather than a unique experience to a unique character, which if how I view a roleplaying adventure.
I guess that's also why I never really replay CRPGs. There are a lot of which I have great memories that I'd love to experience again, and a lot of games I've played in the original version, rather than with the community mods or enhanced edition (I never played TWEE either). I have replayed strategy games like Civilization 2 and Stars! about a million times, and I've played pure tactical games like Nethack and Adom possibly even more. I love RPGs, and that includes CRPGs, more than I love strategy games, yet for some reason I never replay them. I'm not entirely sure why, really. Apparently I'm unique in this.