Forgotten Realms:
Pro: Close to core D&D feel/rules. Very well supported. Rules are precise. Great adventure seeds. Lots of interesting opposition and compontens.
Con: Very pedestrian D&D. The setting is STILL overshadowed by powerful setting NPCs. Despite claims to the contrary, the material for it is NOT balanced with the core.
Kalamar:
Pro: Close to Core D&D rules. Nicely detailed are beleivable locales and cultures. Beautiful atlas and GM screen.
Con: Lackluster to bad rules support (that I am aware of -- I haven't seen any books since before their monster book, but most were pretty bad.) Much material you already have. Somewhat un-fantastic feel.
Scarred Lands:
Pro: Intriguing backdrop that works well with the default D&D assumptions while taking it a few fresh directions. Very well supported. Threatening (and thus nice and tense) feel. Interesting magic rules and creature books (ratmen! hags! tatooed witches! You gotta love it!)
Con: Some rules material is a little shaky. Perhaps too much support -- it's hard to keep up! Maps could be slicker. The "sorceresses must be scantily clad" rule is sort of corny.
Oathbound
Pro: Like ravenloft, you can pull from anywhere. Fresh, intriging backdrop. Lots of racial options. Lots of goodies for powergamers (not that that's a bad thing.)
Con: Many of the rules are questionable (prestige races) to bad (the spell point class rules in Plains of Pennance.) Not for those who like low power.
Midnight
Pros: More dark fantasy, low power setting. Magic has a different feel, perhaps more what you are looking for if you want something like LoTR. Less emphasis on magical bling-bling. Very moody.
Cons: Seems like it might be a bit depressing for what some players are looking for. Big difference in the classes may breed unfamiliarity.
Dragonstar
Pros: Decidedly different, and in places very creative (paladin elite corps along dark elf inquisition.) Answers the question "why aren't dragons in charge." Very well done expansion to the ruleset that works well without altering the basic assumptions too much. Decent third party support. Guns! Spaceships!
Cons: Perhaps too different; some people loathe mixing SF and fantasy. Follow up books are a little lackluster and don't do much to provide the GM with ideas.
Rokugan
Pros: Very detailed setting, different than core rules classes. Nemurai fix the problem of looting bodies while still allowing for magic. Flavorfuls spells & magic (especially blood magic.) Some interesting spins on cosmology. Fantastic presentation and well done rules.
Cons: There is a strong metaplot. The way serious dwells a bit too much on minutia. High entry barrier - at the very least you need OA and Rokugan in addition to the core books, and you probably should get the creatures and magic books in addition at a minimum.
There are others, but for now, those are the ones I have the most to say about right now.
