Worst game I've played? Honestly, probably Brutal Legend.
I was SOOO looking forward to that game. On the surface, I love the premise. You're Jack Black trapped in a heavy-metal world where you get to use the powers of rock to smite your enemies? That sounds several tasty varieties of epic.
The demo lead me on even further, leading me to believe it would be a hack-and-slash type game with an emphasis on team-ups and boss battles.
Then I actually played it. What a freaking bait and switch. After only a few short hours, the game basically turned into a straight-up RTS with limited options and even more limited control. I'm not a huge fan of RTS (except for Starcraft), so this was like drinking a glass of fine wine only to discover it's actually piss.
And before anyone gets on me that, "They said it was going to have RTS in it", that's still no excuse for leading me on with that demo. A demo is supposed to ACCURATELY reflect the gameplay, and Brutal Legend's demo failed so hard in that respect it hurts.
I'll admit that the comedy was good at first, but it quickly got tiresome.
Man, what a letdown.