As others have pointed out, the nature of the changes have been different from edition to edition. But each new edition is grounded in the evolution of the previous.
OD&D to AD&D (and B/X D&D): A period of heavy experimentation followed by a sudden crystallization and desire to stabilize the rules set. This becomes the pattern for all edition changes to follow, but the reversal is particularly stark, and rules changes slow down to a crawl for many years.
AD&D 1 to AD&D2: Desire to broaden the scope of the game to incorporate story, wider themes, and more diverse worlds. Mechanics take a back seat, and a trickle of novels, settings, etc starting with Dragonlance in the late 1E period leads to the 2E flood.
AD&D 2.5: Mechanics do not rest entirely, and after years of near stats there is a wave of mechanical experimentation, in part driven by mechanics in other games, to allow for more detailed skills, more player options, more flexible magic, more tactical combat…
AD&D 2 to D&D 3: The desire is to get back to what made the game so popular in the first place, while ensuring its appeal to a wide audience. A baroque mass of mechanics dating back 25 years is cleaned up, story is given less emphasis, but many campaign settings still receive support, and effort is made to keep a surprising number of details consistent with past editions.
D&D 3.5: Balancing and clarifying options, part of the 3E set of revisions, takes top priority. This is followed by the tweaks to the core rules with increasingly experimental, even radical, rules options in supplements.
D&D 3 to D&D 4: Seeing D&D squeezed between WoW and Settlers of Cataan, improving the game play experience takes a higher priority then since the very early days of the game. The 3E mechanical core is retained, as are familiar options, but otherwise the revision is extensive and deep, to keep the game smoother and more DM friendly while retaining a Titanic load of player options.
D&D 4Essentials: Perhaps the earliest “dial back” after a new edition since Modlvay Basic, this seeks to further push ease of play, while making the game, at least superficially, more similar to past editions, in part out of a semi-belated recognition of a potentially large but also aging fan base.