Dishonourable mentions: Ghost Shark, Scary Movie 5, er... there's probably others I'm missing...
10: Closed for the Season - Firstly, because I can't watch every movie within its release, I'm considering movies of '12 I just watched this year, fair game. Besides they probably premièred somewhere this year, right? As for Closed for the Season, its a character-driven movie, but often includes pointless gory scenes. Usually involving characters dying then reviving. If it was to prevent younger viewers getting bored, then many younger viewers wouldn't enjoy this horror archetype anyway. The movie is about 105 minutes long and many parts are unnecessary.
It's unfortunate, because Closed for the Season does have a genuinely touching story, its just told horribly.
9: I. C. U. - Three people witless, I mean witness a murder from their apartment building while spying on those living in other similar buildings. You'd expect a Rear Window-type story. You'd get a Rear Window-type story without any suspense or characterisation. The first hour often wastes time with video footage of people on the streets of Australia, the killer's video footage of murders, dream sequences, and someone going swimming. On the roof. Of a tall building. During a thunderstorm.
8: Truth or Dare - Basically some guy tricks people he suspects of causing his brother's attempted suicide to a fake party, then drawing them into a sadistic game. Unfortunately the execution can sometimes ruin everything and this movie's final girl earned her place amongst the most detestable characters I've ever seen.
7: Squeal - Pig/human mutants capture and kill them using methods you might expect from a slaughterhouse. But somehow, its just much more boring than you'd expect.
Technically this one's outside the "fair game" category - after watching it I discovered its UK premeire was '11. However, I only continued watching because I released it was August and I didn't have many "worst 10" candidates - I would've probably turned it off otherwise.
If you think it shouldn't qualify, then Ghost Shark is #10 and 10-8 are 9-7 now.
6: Basement - Another group of people go underground where something menaces them. Now, many character-driven horrors can take forty minutes establishing characters. But here, forty minutes in, I realised I couldn't remember the characters doing anything but vapid bickering, so it didn't seem like a character-driven movie. So really just forty minutes of nothing much.
5: You're Next - This home invasion black bothered me primarily because it simply because it wasn't really funny. Many people mistake it for straight horror and frankly, the reputation of sites like IMDb isn't a factor this time. The most common reaction I felt was "huh... I guess that was supposed to be funny".
Also, whenever Larry Fessenden (my hero) appears in a horror film I expect its going to be something special, learning he was in "You're Next" (in the ending credits) just upset me.
4: Hollow - A pretty lazy found footage movie that demonstrates exactly why, no matter how many examples of decent found footage I see, I'm reluctant to watch them. People scream, the cameraman then swings around but whatever was screamed at has gone. That pretty much defines Hollow.
And, like many other found footage movies, you just don't learn much about the characters.
3: Forest of the Damned - Teens in a forest getting stalked by succubus creatures. It's actually difficult to describe why it was so bad in words. The execution wasn't very good I suppose. Additionally, I'd tried it watching it before, years ago, though I don't remember sitting through the whole thing. Yeah, again its technically outside the "fair game" category.
If you think it shouldn't qualify, see Squeal; if you think neither should qualify, then Scary Movie 5 is #10, Ghost Shark is #9, and 10-8 & 6-4 become 8-3.
2: The Inside - See Hollow, except this time, nobody can hold the camera anywhere near straight.
And finally, something I personally consider the worst of the decade so far:
1: The Purge - The concept of people not being required to follow any laws could be disturbing if portrayed realistically. Unfortunately, the writers apparently thought "no law" meant "no emotional or common sense-based reasons for killing whoever mildly annoys you". The failure of the characters to behave and talk like real people combined with the writers' preference for (very short lived) twists over any characterisation ruins the concept.
Congratulations, Platinum Dunes - now anyone with a good version of the concept will get accused of "ripping off" your nonsense!