Well, the following weren't bad, but they could have been better:
Star Wars: The Force Unleashed. The concept was brilliant, the story was excellent, some of the force powers were a joy to use. But there were bugs everywhere, the graphics were pathetic (although the environments were varied), there's no strategy in most of the boss fights, some parts just are not fun.
Assassin's Creed. Very enjoyable, to be sure. But I don't want my experience ruined by some old man who I just killed rambling on for 20 minutes about how he is right, and how I will burn in hell for my foolishness etc. etc. They should have either let me skip those things or made them MUCH shorter.
Then there were the pointless menial tasks that were fun the first couple of times and got annoying the tenth time you have to pick somebody's pocket. The combat was...interesting, and fun when there are a few enemies, perhaps up to ten, but being swarmed by guards EVERY TIME you kill this important tyrant is extremely annoying, and gets boring after a while.
The final (and only) two boss fights were a joke. But jumping from building to building, the assassinations themselves, leaping off a viewpoint were sheer brilliance.
Ninja Gaiden Sigma: I'm sure a lot of people enjoyed this game. I loved it, for a couple of hours. Then liked it, for four hours or so. Then disliked it, and by the time I finally quit, I positively hated it. The game hates you. It despises you. The difficulty curve goes something like this:
It starts off as a really enjoyable hack and slash. Then you begin to see its weaknesses: it's not very varied or particularly gory, the graphics look like a PS2 launch game, the story is pathetic, and the bosses aren't very intuitive. But it's still fun.
As if in response to these thoughts, the game says "screw you", ramps up the difficulty, and makes you destroy a helicopter with a bunch of shurikens. Again, pathetic story. You are a ninja whose tribe is under attack by some crazy super-futuristic gang, with motorcycles, assault rifles, tanks, and as I said, helicopters. It's now that you realise how awful a hack and slash can be, and thank Santa Monica for creating God of War.
(Former) Lead Moderator and (Eternal) VGC Detective 
