What I've regretted buying :
- Mongoose's Dragon Magic. Not what I expected at all and didn't interest me in the least.
- Some Penumbra stuff: the Bestiary (too many Weird/Cute creatures, not enough 'wow! that's spectacularly cool' beasts.
The Monte Cook adventure based on the Half Dragon-Template.
- Kingdom of Kalamar : not bad, but I realised I'd never use it and some of the adventures weren't very good or interesting. I only kept the Villain's Design Handbook.
- Midnidght: it just wasn't for me. The 'You cannot defeat Izrador no matter what you do'-caveat, the weakness of spellcasters and the overabundance of invisble, intangible evil spirits who possessed people or animals killed it for me.
-RttToEE: an interminable, boring dungeon crawl as written (I am in awe of the people who took it apart and rebuilt it as evidenced over at Monte's forum, but it didn't inspire me at all).
-SAS d20: not bad, just will probably never use it, what with M&M, DNW, and Hero...
-Star Wars Tempest Feud adventure
-Bluffside: don't like the history and idea of the city
-Bloody Sands of Sicaris: another adventure with great NPC's and a cool intelligent plot which reduces the characters to the level of bystanders and bodyguards of the REAL main characters. Same could be said to a certain degree of the Witchfire trilogy but there I feel there is enough gold among the dross to keep me interested (though running it would require MAJOR rewriting).
-And some Scarred Lands stuff: the dreadful Penumbral Pentagon, but also Hornsaw Forest (way too much 'gee, look how cool these elves are: they use piercings and tattoos', too reminiscent of the Earthdawn Blood Wood (and inferior to it), a dull adventure which is nothing but a magic-using spider filled dungeon crawl...). I'm not ecstatic about CCIII either (50/50), but don't really regret it.
- And various and sundry modules which are either senseless dungeon crawls or just not very interesting at all.
I will leap to the defense of Masters of Arms because it makes a valiant effort at rendering d20 combat more visual and exciting. I enjoy it a lot.