AugustWolf
This article titled Lion Heart or Snake Eyes?: Percy Weasley on Mugglenet discusses how Percy should be a Slytherin. He's overly ambtious, abandoned his family, etc, showing more and more traits that do not fit the Gryffindors...

Actually, in my opinion, I can draw him back to being a Gryffindor.
Bravery: It took him a lot of courage (and stubbornness) to leave his family and finally live for himself. That takes any boy a lot of courage. He's been getting the dirt wiped off his face by his mother all his life, and it was time he grew up.
Any Gryffindor might hate me for this, but in the books, we have seen Gryffindors constantly show traits of arrogance and stubbornness (especially Harry Potter...). Percy is incredibly both of those.
I'll list some characters that show those bad traits...
Arrogance:
Hermione, Fred + George (Above the rules), James Potter, Young Sirius Black, Cormac Mclaggen, and some could probably say some others are, but they are not that obvious...
Stubbornness:
Harry, Hermione, Ron, Seamus Finnigan, Hagrid, Probably James and Lily, Oliver Wood (About quidditch), Lee Jordan (About announcements for quidditch...), Romila Vane (trying to force Harry on her...). I could probably pick a few more if I was a good enough HP sleuth.

Stubbornness goes right along with Bravery a lot of the time.

I really hope he goes back to his family. I think he will. He won't admit he is wrong (which is the whole reason for all of his nonsense...), but hopefully he'll go back.