Ranked in these QuestionsQuestion Ranking
Pro Compatible with D&D 3.x material
The D&D v3.x family was published under an Open Game License which encouraged third-party material and as a result provided an enormous pool of resources. And two D&D editions later, there is still a thriving market creating new 3.5-compatible resources.
Pro Open gaming with lots of options
The base world is detailed but still open and there are tons of options.
Pro Modular system encourages creativity
The Dsystem is designed around building characters, monsters, magic items, etc., from small simple pieces. For each piece, any of countless others can be substituted, allowing near-infinite customization and variety.
Con Detailed combat simulation slows down gameplay
Due to countless situational modifiers, players and DM alike have to put a great deal of thought into decisions about position, order of actions, etc., to the point that six seconds (one combat round) of "in-game time" frequently takes half an hour or more of play time.
Con Encourages character optimization outside of play
With so many options available to the player, and the almost unlimited ability to combine them, certain "character builds" are patently superior to others. Players who spend a lot of personal time poring over the rules can often create characters so powerful that players who don't optimize feel useless in games with those who do.