After my recent trip to Disneyland/Six Flags Magic Mountain I made a post on my blog that listed my rankings of heavily themed, lightly themed, and unthemed coasters that I've experienced. I haven't been to Six Flags Great America or Cedar Point, but I feel that I've been on most, if not all of the coasters in this country that I would consider heavily themed. With that said, this is the quote from my blog (which is by no means censored, so read at your own risk):
Best Heavily Themed Coaster: Space Mountain (Disneyland, Anaheim, CA). This classic indoor roller coaster welcomed its much needed refurbishment as improved effects and a fantastic musical score makes this the best themed coaster. Space Mountain doesn’t rely on large drops or inversions to generate its thrills. They’re generated by the special effects, soundtrack, and the appearance of speed. Others that received consideration were Expedition Everest at Disney’s Animal Kingdom, both versions of Big Thunder Mountain Railroad (Disneyland and Magic Kingdom), both versions of The Mummy (Universal Florida and Hollywood), and Rock ‘n’ Roller Coaster at Disney’s Hollywood Studios. Space Mountain at the Magic Kingdom (In Orlando) and the Matterhorn in Disneyland did not receive consideration.
Perhaps it's the huge disparity between the two, but I don't consider Space Mountain (in Disney World) a top notch themed coaster any more. I would probably put it at the bottom, or right ahead of the Matterhorn. Any improvement would be welcome, but the most glaring need is to quiet the ride and smooth the track. That would probably bring it up to the caliber of Big Thunder Mountain. It will take improved effects and/or onboard audio for it to even approach Disneyland's version.