It might take me a while to get into this thread, but I'll bite.
Innovation and pushing the boundaries seem to be requirements for you all to say that a band is great, and while that IS a big factor, I think a band can be great without doing either. The Eagles are a great band.
Beyond that, there are many people more musically savvy than myself that would argue that the Beatles were "relentlessly imaginative and experimental" musically. You also have to remember the context of their music. In recent musical history, MTV Unplugged was considered extremely "creative." We're used to hard-driving guitars and guys playing 14 drum kits at once. The electric guitar itself was still a limited phenomenon when the Beatles arrived. Their look, their music, everything was new in 1964...they were as out of the box as you could get at that time.
There is also something to be said for being the first. The Beatles may not have been the absolute first to play similar music, but they WERE the first to bring it to America, where it exploded. Someone mentioned that they were just another British pop band. Well, obviously they were not. Otherwise, we might just as well be talking about Peter & Gordon or the Monkees. The British pop bands copied the Beatles, not the other way round.
Finally, Lennon, McCartney and Harrison WERE great artists. All three are poets of the highest caliber, McCartney's voice is smooth and soothing, and Harrison can play the guitar with anybody.
Personally, I can't sit and listen to an entire Beatles album in one go, but I can't deny their impact on modern rock and roll, nor their talents. Just because they are not my favorite band, doesn't mean they aren't the greatest of all time. For comparison, I'll throw a couple of other names out there, that none of you probably like, yet they are (were) great at what they do. Elvis, Wacko Jacko (either one), Prince, Merle Haggard, Willie Nelson.
The Beatles started the British Invasion, which started the rock and roll revolution.