In my experience, 2e was a serious and unqualified improvement over 1e. I had a lot of fun with 1e, and continue to have great fondness for that system. But mechanically, it was always a bizarre patchwork of weird little subsystems, many of which were broken, and toward the end of 1e the rules had grown overly complex and unwieldy. I also hated -- HATED -- that ole party-line mentality that the one and only way to play D&D was TSR’s Official (tm) way.
By the mid-eighties, several alternate RPG engines had sprung up that were mechanically much superior to 1e. I was very close to porting my existing campaign over to GURPS when 2e hit. And I thought Second Edition was so good that I didn’t look at GURPS again for almost ten years. Zeb Cook and company did a marvelous job of streamlining and consolidated the AD&D morass. And I loved the new, refreshing attitude supporting house rules.
Were there some changes I didn’t like? Sure, but (as with 3e) it seemed like about 90% of the changes were for the better. On the whole, the second edition PH, DMG, and Monstrous Compendium were all good, solid books.
However, I think when most people say they hate Second Edition, they don’t necessarily mean they hate the game engine itself: I think they often mean they hate the way TSR was run or the way Second Edition was implemented during the late eighties and early nineties.
For example, I think those three excellent core books were then followed by a series of terrible (or, at best, inconsistent) splatbooks filled with uninspired and often broken rules. You thought Sword and Fist was pretty bad? Let me tell you, it is nowhere near as bad as the Complete Fighter. I can still remember how disappointed I was when I finally got a chance to read that book.
And for whatever reason (maybe influenced by the Marvel’s Secret Wars and DC’s Crisis on Infinite Earths), the TSR designers chose a very ham-handed way to introduce 2nd Edition rules to Greyhawk and the Forgotten Realms. They went for huge, cataclysmic in-game events to justify the new rules, events that significantly changed both settings, and maybe not always for the better. A lot of these continuity changes drove long-time fans nuts, which is unfortunate since they were probably unnecessary. Instead of just re-statting those Scarlet Brotherhood monks as 2e fighters with an appropriate kit, TSR had to have all monks killed off in one big purge. (OK, I’m only slightly exaggerating here.)
To compound this initial blunder, TSR kept jerking around support for its official settings, adding new lines, dropping old lines, reinstating lines. This really ticked off those gamers who tried to follow one or more of these settings.
So, while I think the 2nd Edition game engine itself was a darn good one, and while there were many outstanding supplements released during the 2e period, inconsistent quality (probably resulting from pushing out too many products) and vacillating support for the official settings turned off a lot of loyal fans.