Sacrred Lands is probably the best setting for 3E because everything actually has a place (well, monks still seem iffy) and doesn't seem to be thrown in like the FRCS with the addition of Sorcerors and the not too well thought out addition of monks. They didn't even make an attempt to use the Salvatore monk ideas that actually made sense. The Scarred Lands was designed for 3E rules and the rules fit like a glove, even without the optional rules from the R&R series etc.
I think what makes it best is that they are just different enough from standard DnD to be interesting and similar enough to keep a DM from getting confused. Drow are not spider worshipping freak jobs, nah, they are cooler than pooh in the Scarred Lands and have the best greeting since Transformers the Movie. I love the Charduni.
The monsters reek of evil, they are dastardly forces from the bowels of Hell and most of them smell funny too.
I also like that alignment seems more real without being restrictive. Evil and good work hand in hand to defeat a greater, more sinister evil. Like was said earlier, they aren't necessarily drinking buddies, but they will do what is necessary to survival.
The setting is not overwhelming either. The Realms just seem overwhelming in their detail and that can intimidate a DM, especially when a player knows more about the setting (and I don't mean just the novels, we don't read them and I have a player that gets pissed off because I changed some aspects of Scardale Town and other things).
Price. Most of all the price of the books makes the Scarred Lands great. They are not overpriced books. I like the Forgotten Realms books, I DM in the Realms, but I prefer the Scarred Lands immensely. The Realms books are the same price as the hardbacks though and too me that is just not easily overlooked. I feel shafted. If they were hardback at 30 bucks I would feel that I paid a good price. Were the hardbacks 10 dollars more at the same size, I wouldn't feel that 30 was too much, but when I see BoVD beside the Lords of Darkness and they are the same size, full color etc and both are around the same price but one is paperback... eh. Scarred Lands all the way baby, excellent for the money and excellent in design.