The unfamiliar valley

In robotics and 3D animations there is a hypothesis called the uncanny valley, which states that people accept robots or 3D avatars that either look exactly like humans, or that are very different from humans, but that they are revulsed by them if they are "almost human". I was wondering whether a similar "unfamiliar" valley exists for MMORPGs, and in how far that valley will hurt Rift.

My theory starts with a personal observation: I do consider Rift to be a perfectly good game, but I feel no desire whatsoever to buy it. I did buy Cataclysm, because it is exactly like World of Warcraft, and I will most probably buy Star Wars: The Old Republic, because it is sufficiently different from World of Warcraft. But for Rift I would have to relearn a lot of stuff, e.g. how the different classes and stats work, and I would have to level up new characters, find a new guild, make new friends, all to finally end up doing pretty much the same as I'm currently doing in World of Warcraft.

Now that could be just me, but I'm wondering if that feeling isn't more common, in view of comments I received on my previous Rift post. Cataclysm famously sold millions of copies in the first 24 hours, indicating people do want "more of the same". But an expansion offers that "more of the same" bundled with familiarity. Cataclysm changed a lot, but not so much that players are now unfamiliar with the new World of Warcraft. A lot of the knowledge of the game they have is still valid, and in spite of the gear reset they still have access to a lot of the committment they invested in World of Warcraft, be that in the form of character development, or in the form of social contacts.

If you switch to a different game, you will be unfamiliar with many things. You might be able to apply some of the knowledge from previous games (e.g. a mage in Rift still pulls mobs from maximum distance of his fireball spell, just like he'd do in WoW), and you might bring a part of your friends or guild over to the new game. But there are certainly things that you will have to learn, because the new game is somewhat different, and your circle of social contacts will be different in the new game as well. You might be willing to accept that unfamiliarity, if in exchange you get a sufficiently different game experience. But are people willing to accept the unfamiliarity just for minor variations of the game experience? Or is there an unfamiliar valley between "more of the same" and "something completely different"?