I'm currently in Durham, NC visiting a friend from school, and so out of necessity, this might be a bit short. This is something of a follow-up to a comment I made over on Grognardia. Put simply, if you don't buy the "evolution" argument about role-playing games, i.e. that games "evolve" somewhat naturally from previous editions, and that the successive editions are somehow "better", then it isn't much more of a stretch to realize that Advanced D&D 1st Edition isn't the only way in which Original D&D could've evolved. You could take this further, and note that the very existence of Holmes/Moldvay/Mentzer D&D pretty much proves this point.
"So? They are still all 'D&D', right?" Well, yes - but that misses an interesting implication. There are a lot of different ways Original D&D could've evolved; Arduin Grimoire is one direction, and Tunnels and Trolls and Chivalry & Sorcery are others. At some point, we cross the line into "new game" territory. Both T&T and C&S are good examples of games straddling the line, or so it seems to me - clearly inspired by D&D, but different emphases than the original three little tan booklets.
I'm one of those guys who likes Original D&D a lot, but I also like some of the crunchy goodness found in the later supplements, particularly Greyhawk and Eldritch Wizardry. Blackmoor is also interesting - so if you go on to look at Dave Arneson's First Fantasy Campaign you get a very different vibe than AD&D. What I am excited by is the prospect of lots of different re-imagining of D&D, without the judgmental label of "fantasy heartbreaker" - the current round of "Original Edition" products is proof that there is a lot of creativity out there; just look at the latest issue of Fight On! for proof if you don't believe me.
Besides all that, though, is the very real note that if you add up Original D&D and its four supplements, you still don't have "AD&D" - you 've got something that resembles AD&D, but is not the same thing. Probably the most important difference, rather than between this or that mechanic or character class, is a difference of tone. AD&D is clearly the "tournament standard" with Gary Gygax's Foreward in the DMG, whilst Original D&D is much looser and open to creativity, even with all of the supplements. And that's a good thing.