As far as products I regret purchasing... let's see here. Let me say up front that I don't have most of these handy, as I only packed up things that I actually use to my dorm room. Thus, anything I actually thought was bad/useless is now sitting in a milk carton in my closet at home, collecting dust. Therefore, I'm working from memory. That said...
I was disappointed with Evil, mostly due to the poor mechanics contained therein. They didn't mesh well with the rest of the 3e system. I don't have the book on hand to give more detailed criticism, but I do recall that I was left with a very bad taste in my mouth after reading it.
The monster book from Bastion Press didn't strike me as terribly exciting. While there were some neat ideas, the cost was too high for what was delivered. Too much of it was things like "cat-people"; "dog-people", etc. IMO, a monster book should have a distinct flavor to it to make it worth purchasing, such as the Creature Collections from S&SS. Those are interesting just to read, let alone use in-game. Also, I'm still not entirely sure why Bastion insists on running everything full-color on glossy paper. Does that change anyone's buying habits?
As has been mentioned before in this thread, the supers game that was released just after the PHB was really bad. I'm still wondering how Foundation got published, and it destroyed any thoughts in my head about d20 being a good supers system for a long time. Recent stuff has renewed that idea, but still... *shudders*. Doubtless this has something to do with the fact that I was running a supers game at the time that the game was released, and was pumped about the genre. Reading this was like a swift kick in the, well, excitement. It's always worse to get hurt when you're excited. (I'll let you reverse-translate that, since Eric's grandmother might be about.)
I was extremely unimpressed with the Deadlands d20 book. First of all, Deadlands is a very cool system itself. Why convert it? It's like making Amber d20, for cryin' out loud. Sure, you could do it, but what's the point? Second, it demonstrated a very poor grasp of d20 rules. The classes weren't balanced against each other - the gunslinger could kill just about anything, thanks mostly to the very poor feat chapter. The huckster lost all its flavor, and couldn't compete with the other PCs due to the crippling nature of magic. The same is true of the Blessed, though to a lesser degree.
The problem with AEG's Deadlands d20 is exacerbated, however, by the AMAZING COOLNESS of their Rokugan stuff. The Rokugan main book is probably the best d20 product I have purchased to date, which covers quite a lot of ground. Every time I open the thing, I'm shocked by how good a job they did with it. And at the same time, I feel a twinge, knowing that Deadlands d20 could have been just as cool. It's like they just didn't care with Deadlands, which is a shame. It's such an amazingly cool setting, and I'm of the opinion that everyone should be exposed to it. That's right, everyone. You should walk down the street waving a copy of the Deadlands Player's Guide at people. It's your duty as a gamer.
I haven't actually been disappointed by anything from WotC, at least not in the long term. Sword and Fist had some pretty severe problems when it was released, but those have been fixed with errata. And honestly, the ideas were there. That's what counts, to me. I can (and usually do) fiddle with mechanics. That's not what I'm buying when I buy something like a splatbook. I'm buying cool ideas.
Song and Silence left me a bit dry on the idea department. The best chapter of that book (by far) was the feats; I've used most of them by now. I was mostly irritated by the lack of bard stuff, as I had just done quite a bit of work on a bardic prestige class or three, and wanted to see some new ideas about bards from WotC. More stuff about trap-dodging, pocket-picking vanilla rogues didn't interest me. The only section that was really bardic was, IMO, poorly handled. Special rules for bard song based on your instrument? Huh?
Well, that post turned out rather long. Perhaps I'm trying to avoid sleep, knowing that I have the first class of the new semester in the morning? Hm. Warcraft 3's an awfully good game, and I could do some more scripting for Neverwinter Nights tonight. I can already tell it's going to be a long, hard semester...