I see folks posted some solid contenders already - Spore messed up on so many levels it's not even funny, and it had some remarkably ambitious stated goals, Hellgate: London had some nice ideas and started out pretty damn fun, but it was riddled with bad design choices as well as gamebreaking bugs. And let's not forget Daikatana - given its pedigree and scope of its ambitions, I were not expecting an unplayable PoS.
I'm kind of surprised nobody mentioned Godus so far - I know, I know, it's Molyneux's game and if there's one thing he's known for, it's overpromising things to the point where it sometimes looks like he just makes up wildly outlandish features on the spot when talking to the press. But then, the guy also earned a reputation for making pretty damn solid games at one point - he had his hand in some pretty amazing strategies back in the day and he's inarguably the godfather of the "god game" strategy subgenre - so, with the most egregious, well known examples of Pete running his mouth being action games (I'm thinking of the Fable series and Magic Carpet), I were thinking the guy might still know how to build a solid, enjoyable strategy.
Boy were I in for a surprise when it turned out "the game of his life" was a glorified cow clicker...
But the biggest disappointment in my gaming life hands down? Deus Ex: Invisible War. I've loved the original Deus Ex to bits, it was one of my most frequently replayed titles and after getting three diffrent endings, I were worried I'd never see a sequel. Thus, I were very pleasantly surprised when I first heard of Invisible War...
I shouldn't have been. The very premise has been a copout, with devs deciding to make an ugly, tangled mess of the three clearly defined and very distinct endings of the original and it mostly just went downhill from there. The new aug system was, IMO, tolerable but uninspired. I understand it was more passive than the original's since it tried to fill the niche left by removing the xp track, but that, in my opinion, was questionable in and of itself. The characters felt sort of bland - the new ones weren't even memorable enough that I'd recall their names by now (there was that young Illuminati couple and I remember some moustache-twirling-villain-flavored twat whom you were chasing after from the very beginning of the game who ended up as a key figure in Knights Templar... Oh and the guy who just sort of went along with anything that happened to him, up to and including getting merged into a Russian shared-consciousness cyborg Mafia) whereas old faces from the previous game had their personalities all messed up. JC was a messianic figure to begin with, but they've overdrawn this in IW to the point where it felt unnatural, for example, and Tracer Tong... I've always liked him, in part for supporting a very idealistic ending in Deus Ex, putting freedom above prosperity. I can understand how he'd grow to change his mind on it, but having the guy who espoused freedom and individuality offer unwavering support to someone who wants to force humanity as a whole to merge into a frickin' hivemind? Come ON! The ridiculous shared ammo system was as unpleasant as it was shallow, making it one of my peeves about the game even in light of all the other flaws competing for attention since I've always had something of a hoarder mentality in games and the first Deus Ex did very well in scratching that itch with all the special ammo types - fun to collect, fun to unload when you finally get in a tough fight! And then of course the coding was a trainwreck, with the bugs having bugged bugs and I can't say I've appreciated the ever-present bloom. All in all, I wouldn't say the game was ALL bad, some elements were passable, but what I got was a far cry from what I had hoped for(pun unintended, I quite liked Far Cry).