On one hand, 2e did an excellent job in terms of taking the confused, inconsistent, and bloated 1st edition ruleset and streamlining and reorganizing it into the three new core rulebooks. I think the 2e PHB still holds up as a solid piece of design. The 2e DMG a little less so, and the Monstrous Compendiums even less than that. But from a game mechanics standpoint, I thought 2e cleaned up a lot of things that I hated in 1st edition, and I greatly enjoyed the shift in tone away from "if you don't run your game exactly like Gary says you should, you're not playing REAL AD&D."
2e also produced some fantastic campaign supplements with great production values. Not so many great adventures, but plenty of excellent settings books that still hold up today. Unfortunately, TSR produced far too many supplements for far too many lines, so the really good books often got submerged beneath the tides of mediocre ones.
I think 2e was marred a terrible series of splatbooks with unbalanced mechanics and no overarching sense of consistent design. I still remember the disappointment of reading the "Complete Fighter's Handbook" with such crappy illustrations, boring copy, and uninspiring design. It felt half-baked and rushed: if the 2e PHB was an A, this thing was a C- at best. This inconsistency across the complete XX line helped tar 2e for many gamers as a broken game for munchkins. Eventually TSR got the message and some of the later books in the line were excellent, but by that time the horse was already out of the barn, so to speak.
The Monstrous Compendium concept of three-ring bound monster sheets was a great idea in theory but far less successful in practice and again plagued by less than stellar execution.
But probably the worst mistake TSR made was to remove demons and devils from the game. Just an incredibly short-sighted move, and by the time TSR realized its mistake it was a little too late. I wept no tears for the removal of the monk, assassin, or half-orc from the game -- I just didn't care for the mechanics and I thought they weren't great thematic fits. But no demons or devils? Come on!
I really enjoyed the 2e epoch, and thought the revisions to the core rules made it a much stronger game engine than 1st edition. 2e kept me playing D&D for 10 more years, rather than jumping over to GURPS or RuneQuest or some other alternative system. But I think the excellent revisions to the core rules and the occasionally great setting books were in many ways overshadowed by an incredibly weak line of splatbooks and some horrible marketing decisions.