and theme are a huge part. They've struggled recently with new rides thanks to the Fire Marshall's ban on so-many people indoor, but they've started to figure out how to work around that. (the que for Carsland still sucks though).
Tower of Terror had the potential to be a classic, a huge part thanks to the theme and que. Essentially it's a covered version of the fall at Lagoon, but it is just so much better taken as a whole. I've heard the Dworld version is much better and it will most likely stay, but it's also on the other side of the coast.
Disneyland doesn't really need to match the "thrill-factor" of a six-flags. They aren't really trying too.