Regarding stuff you don't like for flavor (ie Vrocks advancing): Easy to ignore. Flavor varies from campaign to campaign. They should stick to the standard to a certain degree, but not to the point that they omit options that people might like to have.
Oh, that's that "options, not restrictions" thing. Wow. I like that.
As a DM, I'd probably not let my players use a big portion of what's in the book, most of the time... I can't honestly picture what an Air Elemental PC would be like, and unless a player convinced me it would really be interesting beyond the stat-block, it's not gonna happen. Does that mean they should have cut that out? No. Because when one of my players does convince me an Air Elemental PC would be interesting, I have that option.
A lot of griping was done over inconsistent ECLs for monster races. This book seems to solve this, with few balance issues (the Half-Ogre maybe should have been +2, but let's not go too deep into that.) Further, a lot of people (myself included) were bothered by the fact that some monster races weren't playable at mid- or even lower- levels. I want to play a Mind Flayer because they're fun, and I have no problem dropping some race abilities if it lets me do that... In some cases it may stretch versimilitude, but how-much-is-too-much is again going to vary from game to game. Again, don't use what doesn't fit.
Yeah, this is a boon to a certain kind of munchkin -- not the strict power-gamer, but the sort of person who plays a character just for cool weirdness factor, without respect for versimilitude. You know, the Half-Dragon Kua-Toan Monk/Rogue/Assassin/Deepwood Sniper ("Character background? ... He got kicked out of his pond. And Seeks Revenge.")... Even if that's balanced, how do you DM for a creature (or party of creatures) that makes so little sense?
Of course, in some games that may come close to making sense, or, if it's hack-and-slash, it just doesn't need to. It's nice to have those options are supported. It's nice to have some different, more role-playing or story oriented options supported as well (I've had an itch to run a PbP game centering around evil outsiders seeking redemption for a while now... Makes no sense in the normal D&D canon, but it could be damn interesting, and it's nice to have the crunch available to know you can put together things like this.)
Flavor is not just fluff. But sometimes you need solid mechanics to support that flavor... Especially if you're concerned about balance. SS doesn't outline much flavor, but it gives enough useful, balanced rules that you can try out some flavors you'd miss out on otherwise. It would have been nice if they could have included cultural flavor, and that sort of thing, to accompany the mechanics: Knowing something about gnoll society is useful even if I don't like the gnoll rules, and having some balanced gnoll rules is useful even if I have my own ideas about how gnoll society should be. But then, there's as much about all that in the Monster Manual as I would expect them to fit in a book like that: Larger write-ups would eat into the rules.
If I have to choose between home-brewing my own flavor and home-brewing my own mechanics, I'll stick to flavor any day. Balanced mechanics are beyond my realm of knowledge.
In short, AFAIC this book is everything it needs to be, if not everything I'd like it to be. And that is OK.