Some quick points.
HarnWorld
Pros: Incredibly detailed, Fixed date (no advancement of timeline), Great Maps, very realistic.
Cons: $pendy: Perhaps highest price per page in the industry, Not ruleset specific (not sure if this is good or bad. You won't find D20 rules in here, just descriptions of people. Statting them out shouldn't be difficult). Low Magic.
Personal notes: Harn is my first love. The initial overhead is impressive but supplements come sporadically so it's easy for me to keep up.
Scarred Lands
Okay, I'm baised on this one since I've done writing for them.
Pros: Sort of a gritty heroic d20 game. There is a lot out there for it, but you needn't get everything. Good implementation of some of the core classes.
Cons: The organization of the line is somewhat haphazard (especially considering that the first book out was a monster book). It's smoothed out some recently however. There's enough supporters around here to tell you what the must have books are.
Forgotten Realms
Pros: The Realms has its detractors but there is a lot of support for this setting. The Realms are big and you can probably find room for any sort of setting somewhere in the realms. It also has some interesting options in the way of magic/dieties/races/ what have you.
Cons: Unless you want to have the metaplot, you have to pick and choose what you want to use. Same with the powerful NPCs. But it's easy to ignore the features you don't want. My realms game only had two NPCs over 10th level and they were sort of distant figures (not much screen time).
Dragonlance
I love Dragonlance, loved it since Dragonlance Adventures (1e). That said, I'm incredibly disappointed with the new 3.5 vesion:
Pros: It's Dragonlance, with all that dragonlancy goodness. Draconians, Dragons, Mysterious moon aspected Wizards, Noble Knights. There is a good breadth to the setting so that you can find a lot of room to game in. The setting has a nice, almost post-apoc feel to it.
Cons: Some of the prestige classes aren't well conceived. I couldn't suspend my disbelief for the timeline past the Age of Despair. I wish tinkers gnomes had rules to actually tinker. Maybe there is errata of another book has fixed it, but since picking up the main book I haven't had the desire to look.
Sovereign Stone
Pros: The magic system seems really cool. They have some new base classes if you're into that.
Cons: The base classes don't seem to add much (mages excluded). The setting itself didn't really appeal to me and I had difficulty suspending by disbelief (ork sailors are cool, orks believe they are descended from orca...well, whatever.) In many cases the descriptions of people, settings, and motivations seem to be very two dimensional. There are supplements to this setting which I don't have which might fix some of this.
I'll reiterate that for the last two, I only have the main book so I'm not an expert on the setting.
Werner