Hmm... bland books?
Fast Forward Entertainment seems to be the king of these- Treasure Quests and Enchanted Locations are both boring and munchkinish (and the latter strikes me as completely useless). Though I think Fast Forward has been bashed enough.
The Early WotC splatbooks (Sword and Fist through Masters of the Wild) didn't do much for me, I like FFG's "Path of" books much better. Though it has more to do with balance and flavor than anything else.
Stronghold Builder's Guide and Epic Level Handbook are both dull, but they have their uses. The ELH could have been much, much more...
Hero Builder's Guidebook isn't much of a book for experienced players, but it's good for n00b's.
Unless you're running a campaign based around mortal combat between Gods, I can't see much use for Deities and Demigods, or Faiths and Pantheons. Both missed opportunities- the 2nd edition Forgotten Realms godbooks were much better, as they concentrated on the clergies, temples, rituals, dogma, and politics of the religions, rather than stats for gods (even if the "Specialty Priests" were highly unbalanced, I still liked the books).
Kalamar struck me as kind of "meh"... low magic Faerun, or maybe an alternative to Greyhawk, but nothing that caught my interest.
Midnight looks much cooler than it really is. In play, it's about as much fun as adventuring in Stalin's Soviet Union, about as depressing as Call of Cthulhu, and quickly loses it's charm. Still the best Tolkien knock-off this side of Harn, though.
Excalibur, dull? I got a different impression- the book gave me everything I needed to run a high chivalry game. Sure, they left in the Fae, and made it slightly higher magic than some purists would like, but that adds to it's charm. The personal setting I've made with it is "Once and Future King" meets "Lord of the Rings", and I've got my players quite intrigued. It's the flavor I've wanted since 3e came out- down with dungeonpunk! (pace, Eberron fans)
Mongoose seems to mass-produce lackluster books, though they seem to have plenty of fans. I'm somewhat intrigued by their OGL Ancients, OGL Cyberpunk, and their Birthright knockoff... I might have to pick them up when my Excalibur campaign comes to a close.
I'm mixed on d20 Starwars. It's a good system, and I enjoy playing and running it from time to time, but the mood is quite different from d6 Star Wars. d6 Star Wars focused on the rebellion, and more on "mundane" characters than Force-users. It had much better starship combat, for instance. d20, on the other hand, is very Jedi-centric, in line with the Prequel movies. Each to their own, really.
Well, rant off. There's good, bad, boring, and flavorful in d20...