Well, I am disappointed by D&D books for quite a long time. And I bought just a few of them, core rulebooks and about three or four other. What I really dislike is the fact, there is just nothing interesting, nothing creative in them.
I as a DM want consistent world with functional ecology, economy, politics and so on. And what I get are just inconsisten fragments don't fitting together. Forgotten Realms, however it is nice setting are a good example of this approach. One million inteligent races, one million nations and countries and no connection between them.
It is said, that when butterfly flutter his wings, there is the storm elsewhere. But when in these "fantasy world" there is cathastrophy, most of the time, even the closest neighbours don't care.
We play in our own world. With just three big nations and several small city states and some tribes, because we don't need more for good, long exciting campaign. There are just few intelligent races (even less then in PHB) and there are just few well known monsters, which fit into the setting. We do not use more feats, more prestige classes, more spells, domains, magic items etc. because we do not need them for nice, thrilling and breathtaking campaign.
This game is not about rolling dice and arithemtic it is about roleplaying and none of DnD books helps with this problem. There are no ideas for really good plots, interesting and elaborated persons, completely mapped and described intresting places. This is what I miss in DnD books, although rules are fine, I do not need pregenerated statistics and tables. Even my trained chimp coud make them. So this is I am disappointed by DnD and d20 books. (Yes, I know, there are few exceptions - I bought them, but most of the books are just crap.)
Sorry for mistakes, I am not native speaker. :-(