I recall Gary was already working on a second edition while in California dealing with the cartoon, and was givien his walking papers before he got to do much and then 2nd was made without him. That is what some peoples gripes are.
Others I here that never played 2nd just say it was so bad that caused WotC to make 3rd....
For me and many I know I agree that 2nd edition AD^D was about the best thing made. 1st had the prose of Gary, but also had the excesive wordiness and bad book organization.
2nd, especially revised, had much better organization. They set out to NOT be strict rules, but just guidelines for creating your own game and said so right int he books to stop all the rules lawyer interruptions. This of course didn't help the RPGA set rules when things were different everywhere.
THAC0 was simple and the easiest system to hit with. Also it didn't deny anyone from being able to hit anything because it had a limit to what protection you could have even from magic and made the game more, to steal a word from 4th design, cenimatic, in that there is always a chance you could hit something rather than having things be futitle.
NWPs were elegant, had rules, and were optional. My favorite example to explain this was with fire-building. Not using the rules for a limit to NWPs but to cover instances of where you may have difficulty was a player telling me why there character would be able to perform this act. If they could give a good enough reason then they could build fires, but if they didn't want to know how to build a fire they didn't have to. Not everyone in the real world can make a proper cooking fire!
You even had roles outsides of the acting like another peson in 2nd. You role was the fighter, the mage, the cleric, the thief (your the burglar go burgle something!)
The only reason I have found around me for people that didn't like 2nd was that that didn't like D&D. Oddly they play 3rd or even 4th now and even claim it isn't like D&D but another game, and some still don't play either of those because they just don't like D&D.
I can never find a valid reason especially for the problems people had with THAC0. 90% of the time it involves the negative AC numbers being the problem, yet those people have no problem with any modifiers being a negative making it a penalty to hit rather than a bonus. I mean it is all subtraction.
2nd was best because it just organized 1st up into better reading books for the rules but kept all the good of 1st and added a few clarifications.
I will have to dig out my original 2nd edition books and see what really changed form them and revised, but don't think it was anything but an errata update and a face lift.
It is still the longest lasting edition of D&D at 11 years in production, so I don't know how it could be considered by any the worst edition or RHSC.