I don't think anything besides NaNoWriMo has this much ability to mess with my head. When I'm having a good day, it's like I'm high and giddy; a bad day, and you can practically see the little cloud over my head. It also makes me use more punctuation and similes. I've been whipping out hyphens and semi-colons left and right recently.
It's a beautiful, beautiful thing. Especially the semi-colon. I'm not quite sure I'm using it right, but I can't stop.
My novel this year contains a lot of what I wanted to put in my novel last year. Silly, shoujo-manga-esque things. People walking in at inopportune moments, lots of silly mushy things, and a decent bit of slapstick (mainly involving pillows).
At the same time, I've made it more mature than what I usually write. The main love interest is not some beefcake-y stud muffin. I've described him as pudgy and and cute, and he tends to babble and stumble over his words. (Admittedly, when he does go poetic he does it fantastically.) I'm thinking about having the main character decide to toss away all the responsibility she's expected to take on and run off. Also, she does things like kissing first and blatantly hitting on her cute and pudgy guy. Oh, and a bunch of the other rulers of the kingdoms are completely irresponsible as well. One of 'em has basically decide it's 24/7 party time in his kingdom and left it to his ministers to do all the actually work (while he does the "wine, women, and song" routine).
The one bad thing I've found about NaNoWriMo is that it makes me compulsively hit Ctrl + W (word count in my typing program) which closes Firefox. I already closed this window twice. Ah, the insanity. I'm loving it. (Eat it, McDonald's -- I can spell!)