Since we're on a Zelda forum, I'll go for Skyward Sword first of all. Was very disappointed in that game and was probably the first Zelda game I didn't particularly enjoy. After Twilight Princess, I had very high expectations for a Zelda game made specifically for the Wii and I was definitely riding the hype train during the run-up to release and yet was treated with a game I feel is weak, let alone a weak entry into the Zelda franchise. Overall a poorly executed game imo, a lot of bad design decisions. I disliked all the recycling and back-tracking which was un-necessary, they could have had more variety too, and the justification for it all and the challanges felt unimaginative. Lack of exploration sucked too, and of course the motion controls, I disliked them, for many reasons I'll not bother with. Following on from Wind Waker and then Twilight Princess, a big step down imo. My playing also went Witcher 2 and then Skyward Sword. That didn't bode well for SS, to be compared to that.
Another title would have to be Left 4 Dead 2. I still love you Valve and I'm sorry I had to do this, but, L4D2 was very dissapointing. I wasn't one of the boycotters complaining it was releasing a year later, if it's a quality game then why wouldn't I want it now? And in some places, you did succeed. The physics and the dynamic zombie models were amazing, pipe bombs exploding just became an absolute spectacle. But marred by some disappointing design decisions I didn't expect from Valve. The fluid, amazing animations were gone and wooden, stiff ones placed onto a less interesting cast, and bringing back Francis, Bill, Zoey and Louis made them look even worse by comparison. Disliked the music too, and the new levels just weren't as good, and melee fatigue in campaign? The new special infected were great but I felt the game also got a bit easy, expert mode was way more forgiving. Although Realism helped a lot. Overall not at all a bad game, loved it for Versus, but, not as good as the original.
And, while definitely not a bad game, I'll have to go with Resident Evil 5. I liked a lot of the elements it brought, the inventory screen was a nice hark-back to the old days and helped with the co-op though looking after AI Sheva's inventory got tedious (WHY DID YOU DROP 30 SHOTGUN SHELLS FOR RIFLE AMMO WHEN YOU DON'T HAVE A RIFLE SHEVA?!!) but I felt it drew on Resident Evil 4 a bit too much. Hold up in a house while things attacked, chainsaw man attacks, and I felt these were executed worse too. And since it wasn't new, it didn't have that same feeling. I'd just played an entire game as Leon and killed hundreds of these things, to keep the same atmosphere they had to go a step further but I don't feel Capcom did. I also still don't like what happened to Wesker, but before the ending he was superb. Really on top form, DC Douglas was fantastic as Wesker.
Metal Gear Solid 4, too. Previous MGS games I'd really enjoyed, specially MGS3, and so I was riding high for MGS4 to come out. But, it really didn't impress me. Storyline wise, meh. I thought it was well written, I liked the characters, Raiden especially I really liked, but overall I just felt the storyline could have been executed so much better. I mean, the amount of cutscenes that game had, which did test my patience by the end, I felt like Kojima was beating me over the head with the script shouting "APPRECIATE THIS!", but I just didn't. I'm sure people more invested in the MGS story found it fine, but, I found it all very convoluted and schizophrenic in tone, and a lot could have been shaved off to leave it more refined. Gameplay-wise, it was alright, I didn't use like, more than half of what was at my disposal, but, I felt it only suffered in comparison so I won't fault it for that. After playing DX:HR, for instance, the wall sticking of old MGS especially just feels awful.
And gotta give the final mention to Halo: Reach. As a game, awesome. Enjoyed it. As Bungie's farewell to their series, their swan song, their labour of love that they made it out to be... I just don't see it. It didn't live up to all they were holding it up to, and then 343 came out with Halo 4 which I felt was better than Reach in pretty much everyway. So, I was a little bit disappointed in Bungie for Reach, as I said I still enjoyed it and would recommend it but it didn't live up to the very high expectations that were set out.