I started playing with the 81 Baic/Expert set (interestingly enough, using those two rulebooks in the AD&D S3: Expedition to the Barrier Peaks). I've never looked inside the "original" D&D (except at a download of the blackmoor supplement I saw on a website somewhere). But, here are my thoughts (I don't know if it will have a point, I may just ramble)...
D&D was a nice simple game, which left a lot to the imagination. Dwarves were warriors, elves were magical, and humans could be whatever they wanted to be. The Rules Cyclopedia was a great compilation and a one-book gamers dream (and finally gave D&D a paladin).
AD&D is what I think of when someone mentions "D&D". It has paladins (the one class I'm most partial to playing) rangers, and barbarians, had the separate race/class with limits, and law/chaos and good/evil alignments. Limited race/class combos kept the flavor of the races, and it was nice to hammer down characters. I first discovered AD&D because of the Monster Manual. The biggest decision in character creation was whether to multi-class.
2nd Edition AD&D is what I think of as the best conceived version of D&D. It incorporated the flavor of AD&D1, incuded the proficiencies (which I liked), and made clerical spell availability dependent on which diety you worshipped (why would Vecna care about healing someone?). Wizards could specialize and be different. I missed the barbarian and cavalier, although the kits gave a nice flavor (but started to get munchkin/silly as more Complete whatever books came out, and they started to become too many, but not as bad as 3E).
D&D3.x certainly gave us the most coherent rules system where things worked together. However, in doing so, the game lost a lot of the flavor of previous editions (dwarf wizards??? I thought dwarves were inherently magic resistant. How the you-know-what can they channel magic if they're resistant?). It was nice to have options to customize your character. Then, the expansion books began.. and continued.. and continued.. and continued.. and.. well, you get the idea. Our group is starting a new campaign, and we have like 25 different books to pull races, classes, and feats out of. Overload! Overload!
So, what do I think of original D&D? I miss the simple days. Back then, it was about creating a character in 20 minutes, and getting on to the adventure, rather than spending hours just trying to pick a class combination to build.
To quote Archie and Edith, "Those were the days!"
Eric