I don't love it. In fact I don't think I've been back on it since my then-gf-now-wife and I first went in 2004.
It's easy for me to throw around other people's money, but I think it's the sort of ride that should be updated on a regular basis. So I hold no allegiance to it the way I might for other attractions that feel more "classic" to me. GMR doesn't feel like a classic so much as a celebration of things that are classic.
How I'd change it: Instead of that perpetual loop of clips in the pre-show, I'd set up smaller screens along the way that salute different genres, and rearrange the queue with dividing walls so that a: audio from one screen doesn't bleed through to the next and b: people who are unfamiliar with the experience won't know what's coming next. Each subsection of the queue would celebrate different eras and different genres, different technological breakthroughs. It could be roughly chronological, or each screen could be chronological within a genre (history of animation, history of physical comedy, suspense/thrillers, etc). This would afford Disney the luxury of name-dropping certain movies, even if they couldn't get the rights to use clips or trailers, so it doesn't seem like the history they're presenting is skewed to only celebrate the movies they could get as the best movies ever, but representative of some of the best movies ever. They could name-check Hitchcock without dealing with the headache of licensing clips from Universal, for example.
As for the ride itself, and this is where I commit blasphemy, but I'd set it up to be a combination motion simulator and actual moving ride. Something akin to Spiderman over at IoA, with the sensibility of Fantasmic!. Maybe the gist - which may appeal to you kids with your internets and your youtubes and all - is that you're stuck with 2 people trying to decide on a movie to download/stream on itunes (Aah, sweet, sweet product placement). A fight breaks out, something Tron-like happens, and you get sucked into this virtual world where movies coexist and interact with each other. From there you and these two friends are trying to find their way out, and wind up interacting with recreations of famous movie scenes recreated with a combo of actual movie footage, CGI-recreations and maybe, depending on how old the movies are, even new appearances from people who appeared in those movies, interacting with these intruders (think the commercials that would bring back Sigourney Weaver fighting Alien, or Kathy Bates about to wallop James Caan's legs in Misery, while touting DirecTV).
The best thing about this approach is that, if Imagineers were to plan ahead, it could even conceivably be a modular attraction, making it easier to update, swap out certain movies or certain genres for others - perhaps even allowing a majority rule situation in your vehicle to decide some of the movies or genres that you'll experience. This would also afford guests the luxury of going on again and again and again and never getting the exact same experience twice. And at the end in the gift shop, aside from typical souvenirs of copies of movie props, soundtrack CDs and t-shirts, they cold have itunes-stations set up where you could buy or rent movies (perhaps with a WDW discount) and the ability to send links to the download to your email address at home, an impulse buy that has the added benefit of perhaps exposing people to classic movies they might not otherwise think to watch. And without the danger of forgetting all about the movie 30 seconds after you leave the pavilion (I bet a lot of people even on this site has done something like leave ToT with a desire to track down some Twilight Zone episodes, or get off RnRC with a hankering for an Aerosmith CD, and that urge passes long before you ever get home, and you don't want to pay Disney prices for something you can get on Amazon or itunes, so you forget all about it. This affords you the luxury of getting a movie right away, while the urge is fresh, not having to worry about whether or not its in stock, an option to rent instead of buy so it doesn't bust your budget on a vacation...seems win win to me)
aaaand then Disney pays me a million dollars for my idea, I buy a bunch of DVC points and book a party at the new and improved GMR one night and invite you all so you'll stop hating me for ruining your favorite ride.