Painkiller - Got it (a few months before the ZP review) after reading through the entire archives of a few gaming comics and seeing just how many referred to it. When I'd heard about the weapons, and even the general premise, I thought HELL YES! Went and found it for $10(AUD). There might be a gun that shoots shurikens and lightning, but the completely boring and repetitive gameplay, poor AI, lack of a challenge and shite environments just disappointed me.
Dungeon Keeper II - so many people rave about this series being a complete change in the RTS genre that I had to give it a try. Huge let down. It's inventive and fun, a lot of interesting humour from the Narrator, but again poor gameplay, no challenge, repetition. You're not a Dungeon Keeper, but the director of a preschool play. You can tell the kids, or evil things, where to go, and even take control of one directly, bringing a few along in the process. But trying to arrange specific tasks, or micromanage effectively to override stupid AI is just impossible.
Grand Theft Auto 1 & 2 - Free download off the Rockstar site, figured I'd give the games that started my favourite series a go. Could not possibly be more disappointed. Poor handling and control, pathetic gameplay, boring missions.
I know there's more, I've been on a classic game binge lately and have come across so many disappointments that I'd considered making this thread myself. I'll remember them later I'm sure. Most of the disappointing games seem to be older ones, classics which people enjoy and still remember either because A) it's what they grew up with so they're biased or B) they haven't actually played them in a while, and so all they're remembering are the good bits.