I certainly see a clear progression within classic D&D. A very slow, almost stately progression, I guess you could say. 1e started in 1977, and 2e didn't start until 1989, and was /barely/ different, at first. A lot of years with not a lot of change. Half way through 2e I gave up on it. It just wasn't providing anything of value. I missed out on the 'option' series, and, so I've heard, there were hints at what 3e would go on to do in there. But, I missed them, so to me 3e seemed fairly radical, and a definite improvement.
This millennium, D&D has moved a lot faster. 3.0 to 3.5 to 4e to Essentials in 10 years. 3.5, at 5 years, had the longest run, and it was never stagnant, new spells, feats, PrCs, and so forth were constantly coming out, and changes were made to basic rules and structure here and there, like the introduction of re-training late in the run, for instance. 4e may seem like a radical change from 3e, but given the pace of change within 3e, itself, it really wasn't. Both are merely the modern face of D&D.
