I'm just wondering what games that you thought were easy to finish, uexpectedly easy, or unexpectdly a pain-in-the-arse.
I have to say Fable I and II was the most easy titles to play. The combba system was very easy to handle and the enemies were how to say, very little to worry about. (Even balverines).
Also Mass Effect was pretty easy once you get the spectre weapons and two frictionless material mods. No overheat what so ever.

With cheatcodes or without?.... ;p

The graveyard.

World of Goo was pretty disappointingly easy. The extra optional challenges on each level are probably more difficult, I suppose, but I don't really get into metagame-type stuff like that.

Prince of Persia. Don't Shit Your Pants. Burn The Rope.

It was a bit hard to me at start, but I had no trouble at all with the latter levels. I guess the game turned easy to me once I understanded how the mechanics worked. Those OCD challenges are kinda hard though.
One game that was definitely easy to me was The Legend of Zelda: Twilight Princess. I'm not a fan of the Zelda games and have only played a few, but this one really suffered because of the difficult. It's a shame, really, since the design of the monsters and the world itself is pretty cool. Demon's Soul seems to be the opposite in terms of difficult.

+1
Edit: Installed Medal of Honour: Airboune yesterday morning and finished it by the eveving (was playing it on and off) each mission campaign took about 30 mins to complete (out of 6). The game is way too short, but gameplay is really good.
Prince of Persia also is a worthy mention (as it already has been). Just a tad easy lets say. However I never thought that that was the point. The cliché but epic story and platforming was just the ticket.

I found Ghostbusters laughably easy up till the end then you hit a massive wall of difficulty. Batman: Arkham Asylum is also quite easy on your second playthrough as you know the timings for the combos and enemies spawn in the same places.

Bioshock is really easy to complete too.

Call of Duty 4. Finished it in one sitting. Maybe four hours.

Oh c'mon, that's not a game :-P

Big Daddies were not even hard. 6 hours to complete.
Every Halo game.
Splinter Cell: Chaos Theory is a combination of both, since some levels are harder than others (Epic moment: Tricking a guard into walking near the lighthouse railing and throwing him off as his dying screams land on the grounds.).
Fable: I hated this game and it was wayyyyyyyy to easy.
Left 4 Dead: Since I have a case of OCD when playing certain games (This one was for achievements.), I had a mixed difficulty depending if the game froze or I somehow died in a black room (Literally, a Black Room).
Soldiers: Heroes of WW2: Epic is the only way to describe the difficulty of this masterpiece.

Chains is pretty easy, so is Plants vs Zombies. But then they're basically casual games so you're not going to get a huge level of difficulty.
Bioshock is easy, but it's pretty long compared to many FPS out there. If only they made it more challenging it would have been a pretty amazing game.

Mine: Splatterhouse, Mortal Kombat (with Sub-Zero, at least).

Fallout: Brotherhood of Steel
The game is so poorly balanced that if you played BGDA/DIablo games, it stops being fun an hour or two into it. Great concept, terrible execution. I don't even know if it's possible to die.
They're not easy games on the whole, but I can speedrun most of the Silent Hill games. I can beat them in an hour or so. Ditto the Clock Tower games. If you know what you're doing, you can beat the first one in about 20-30 minutes.