It allowed a lot of customization, while offering depth in traditional archetypes. Niche protection was great, in that every class had something other classes just couldn't do. For instance, fighters just had their d10, good attack progression, and weapon spec, but since everyone else had lower hit points, wore progression, and only dealt base damage, they could always feel good about what they did. There was nothing shameful about being a human fighter with Str 13. You had cleric variants, rogue skill customization, some nice race variants and humanoid options, even custom spell lists if you dug deep enough.
There were things about 2e that really sucked, liked the beginnings of the dilution of the bard, the bowdlerized demons and devils, and the sense that dragons had to be continually escalated to be bigger than the most powerful PCs (I always ask, what is wrong with ordinary, non-planar dragons become old news by the teen levels?). But there were good things, too. Coherence, flexibility, modularity. It's very easy to see how 2e became 3e.