The best part of SCAG was that as soon as I got it, both the campaign I'm running and the campaign I'm playing in had the majority of their characters be modified (either in minor or significant ways), or be replaced entirely, by options presented in the book. The fact that happened showed both the utility and the value of the book.
Specific things I liked:
I liked the very nice run-down of the various gods of the setting - each entry was just about the perfect length, giving a good overview without getting bogged down in detail. And the holy symbol artwork that accompanied them was very well done. I liked that the non-human deities weren't just consigned to lists, but got extra (though brief) descriptions later in the book in the sections on each race.
I liked the "unreliable narrator" vibe of the geography section. Has that gnome sailor actually been to Evermeet, or is it a sailor's tall tale (note that the whole section has nary a actual hard detail of what she saw in Evermeet)? Are the Zhentarim of Darkhold and the realm of Elturgard really as nice as that elf ranger reported, or did she naively ignore (or was steered away from) some of their potentially less savory aspects? I'd love to see the rest of the setting described in such a way in potential future Adventurer's Guides (come on an Adventurer's Guide covering the greater AL region, i.e north-central Faerun!).
I liked getting the most comprehensive update on the Moonshaes since the FR2 accessory in freakin' 1987!
I very much want a decent update on the Old Empires now - not only haven't they been detailed in decades, there are some really interesting new dynamics there now. Although I'm by no means fond of the 4e changes to the setting, I'm glad they kept part of Tymanther around in the region.
And I very much enjoyed the entire final section of the book on races, classes, and backgrounds. As I said, I, and the people I play the game with, have already gotten their money worth just from these chapters alone.
