D&D 3e, 3.5, Pathfinder... What a great edition!
The main problem, in my opinion, of both BECMI and AD&D2e was inconsistency. The lack of a coherent frame led to a proliferation of rules for particular cases. It was a mess, seriously.
D&D3e solved the problem in a brilliant way. The d20 system was coherent, precise, efficient. I remember when I was reading the PHB 3e for the first time: I was fascinated by the new concept of feats and by the skill system (by the way, this was the first time I came into contact with a skill system that actually worked), as well as Todd Lockwood's art. And it worked! It was STILL Dungeons and Dragons, the original feeling of the game was still there!
Later, a new aspect emerged: the system encouraged power-playing, and the study of character builds to make stronger and stronger heroes. That became a mini-game in itself: almost half of the fun of the game was in the character building.
Speaking of the products, the Dungeon magazine (under Paizo) reached its peak, with high quality adventures. In particular, the Adventure Path formula was incredibly successful. Age of Worms AP and Savage Tide AP were exceptional products, love letters to the game and its history; in my opinion, they're the best adventures in the history of the game.
Official products were of variabile quality, but some were exquisite: the Forgotten Realms Campaign Setting is a staple, as the Eberron book; or Tome of Battle - Book of the Nine Swords, that experimented with new and fantastic ideas.
The open license allowed for 3rd party producers to create a great deal of additional material (I want to remember at least the Scarred Lands setting from Seord&Sorcery)
When WotC decided to go 4e, and revoke Paizo's license, I remember my disgust: it was too early, the edition was at its peak, Paizo was doing an incredible job! I wanted other APs! So I jumped over to Pathfinder. I could enjoy Curse of the Crimson Throne, Kingmaker, Reign of Winter and other great stories (even if no one surpassed the Dungeon Magazine's ones; and I despised Golarion ss a setting...)
Eventually, the problem of the edition became clear: it was too complex. It seemed that a degree in math was necessary to play; combats lasted at least one hour and we always forgot something; level advancement was a nightmare, and magic item management too... The game had become rollplay, not roleplay (a consequence of the min-maxing mindset). Moreover, the core rules were heavily unbalanced: some classes were too strong compared to others (the caster supremacy problem).
So in the end, it was tiring. Even enthusiasts like me wanted more simplicity (that also contributed to split the fanbase with the Old School Renaissance movement).
I loved 3e, but I'm really happy that 5e solved all its problems, keeping the better aspects of all the previous editions of the game.