my gf bought me a second controller for my 360 at xmas with the intention to play some co-op games with me. 
naturally i thought this would be a great oppurtunity to bond over some zombie pwnage on l4d2, and then we encountered the witch...
 
my gf shrieks, puts the controller down and says 'i dont like this game its too scary!'
she is the only person i have heard of to be too scared to play l4d2! ive pleaded with her to reconsider and jump back in but she refuses! 
the only other co-op game i have is mw2 and she doesnt like guns... therefore the spare controller is redundant.
fail.

That takes care of that.

The first was a bit scary for me when I first started playing it. No lie.

I thought the first Left 4 Dead was a little scary when it first came out, but not even close enough to make me stop playing it. My main co-op partner for Left 4 Dead 2 was a girl, and she thought the witch was cool, though a little scary as well.

I actually tried to get my gf to play with me about a month ago. She was not used to using the controller and was looking at the sky. She had on my headphones and when looked down and all the zombies were around her making noises she screamed, threw down my headphones and almost started crying. Girls can be different...

yeahh its been difficult gettin her to get into gaming. i bought her beautiful katamari and viva pinata: TIP and she loves those two! 
weve been together 3 years and shes only just getting into gaming! its quite funny rlly but shes obsessed with TIP. 
 
i did have her a split screen custom match of halo3 and it was hilarious she was looking down at the ground spinning round in circles firing! 
i find it strange that some people in this world have NEVER played an FPS... but thats cos ive been playing them since i was 6 and im an ubernerd

L4D2 is more comedic than scary.  I laugh most of the time I play it.  And how could she be fine with zombies exploding but not a little witch?

I don't think Left 4 Dead is really supposed to be scary.  It has zombies, but I think it's just supposed to be a fun action game.  It annoys me when people call it a horror game.

XDD how could she be scared with that? can't think of a 360 game that is cooperative and that she would like.

I'm just looking at my personal library here, but The Simpsons Game was pretty fun co-op with my younger bro. Not to say your girlfriend is like a younger boy, but you know what I mean. I hear the LEGO Star Wars games are cool. You could try those. Castle Crashers is pretty simple and fun. I play that with my brother too. But...nothing else is coming to mind.

The spitter is pretty freaky looking.
 
 http://gza.gameriot.com/content/images/view_320200_1_1248413637.jpg

"The spitter is pretty freaky looking.  http://gza.gameriot.com/content/images/view_320200_1_1248413637.jpg"

Yeah i know the feeling. My g/f HATES L4D2 for the same reason. Dont know what it is, it just gets to some females I guess. She also hated RE5 for the same sorta deal. She was ok with MW2 and the Halos, Gears Freaks her a little but at least she will stay in the room.
 
PS: She loves Castle Crashers I'd say give that a go.