Menzoberrenzan isn't so much "system neutral" as "edition neutral". I'm not quibbling that it isn't good...just that it's clearly for D&D.
I actually think that the same is true for
Pirates Guide to Freeport. While it's supposedly system neutral, it's very clear from a number of things core to the setting that it's a D&D setting, and D&D classes, races and conventions are very prominently on display. In addition, other than the Savage Worlds companion book, none of the others stray very far from the D&D orbit of games--Pathfinder, d20, True20, Castles and Crusades, and I think maybe ERP did a 4e version too. I often compare Freeport to Five Fingers, and while Five Fingers is indisputably a d20 D&D product, it actually feels much
less like D&D in many respects than the supposedly system neutral Freeport, which has all kinds of things that shout, "D&D!" very loudly heavily integrated and embedded all throughout the setting.
I think one of the reasons it works is because they know their market. System neutral really means that it's rather easily useable with a variety of mostly closely related systems that share the same basic assumptions. Go too far beyond that, and you'll find it more difficult to use as is. That said, Green Ronin also included a nice chapter near the end of the book with a lot of different suggestions on adapting the setting material to different basic setting assumptions, which helps make it more truly system neutral.
I suppose that in the context offered here, especially using Menzoberranzan as an example, the same is true. We're not really talking about system neutral so muc as we are edition neutral, i.e., a product that can be used with a game that's transparently the same, or meant to be similar to, an edition of D&D.
I actually think that, in terms of the question posed by the thread overall, what makes a good setting supplement in general? The same things make a good system neutral product. I actually don't much like a lot of stats in my setting books anyway. I mean, sure, I'll take a few setting specific monsters, classes, races or feats or whatever in my D&D games, but if there's very much of that in the book, then I find its utility greatly reduced. I've got a lot of that to choose from already, and want that kind of stuff to be pretty light. The things I tend to like about setting books tend to have nothing to do with the system used anyway, so a good setting book is a good setting book in spite of, rather than because of, the system used.
