See, I'd agree with the tenets of sanj's description and argue that it is pretty much a solid statement of why Qui-Gon isn't a Hufflepuff: sure, he's tenacious like a badger or whatever, but he's also a deeply nonconformist individual. In TPM, he practically double-dog-dares his so-called sett to cast him out. He has no fears about being abandoned or exposed; Qui-Gon practically invites it. While he might "avoid conflict" by using the mindtrick all over the place, I don't get the sense that he has a deep, emotional horror of conflict or whatever. And as, um, we see with Boss Nass, the Council, and Jar Jar (the tongue-grabbing scene), he really isn't very patient at all.

The deal-breaker for me is actually the way that Qui-Gon treats Obi-Wan at the end of TPM. Sure, they kiss and make up in the end, but holy God, the rapidity and SUDDENNESS with which he dumps Obi-Wan in that one Council scene convinced me that personal connections aren't really all that important to him.

Personally, if I absolutely had to put Qui-Gon into one of the houses, I'd say Gryffindor. IMO, he's a lot like both Potter and Black in that they are going to do what they think is right DAMNED be what other people think/what the rules say/what is reasonably prudent. And, you know, if you look at it from another angle, Qui-Gon is kind of altruistic. Sure, he's not necessarily working for the benefit of others, but he is working for the Force.