Okay, serious answer:
I spent about a year in the right seat with my former airline, and now almost two years in the left with my current one. Granted, I mostly fly single-pilot where I'm at now, but we do get paired up with FOs on a regular basis.
From my perspective, I'd say the best trait for an FO is to be assertive when you need to be. Unless the guy in the left seat lacks so much confidence that he can't take any sort of correction from the FO, you're not going to have a problem bringing up ideas or pointing out deviations. In fact, it's expected. This is 2010, not 1946. Jokes aside, an FO's role isn't to sit in the right seat and pull gear. It's to be an asset in the cockpit.
My company has a bit of an issue in terms of CRM with two-crew operations. I'm not sure where it stems from...the training department does train CRM, we have manuals that lay it out very clearly, and yet, I constantly fly with FOs who seem to believe that they're not allowed to question anything I do. Even though the first thing I say when I'm flying with a new FO is "be assertive!", many of them are not. I'm actually on the union safety committee, and I'm determined to address this.
Case in point: Recently I was flying with an FO who'd been with the company for several months. Seemed to have a good grasp about what was going on, and was really progressing well in terms of flying the airplane, planning, etc. Then of course, one day comes along where I'm not paying attention and I bust a taxi clearance. I realize it, stop, and say to him "Did ground tell us to hold short back there?"
His response: "Yep."
He watched me do it, knowing all the while that I was busting the clearance. He just didn't want to speak up, even though I really do my best to create an open cockpit environment. Maybe it's my fault, I dunno.
I'll say this, though: I'd much rather fly single-pilot with my autopilot than with an FO who's not going to speak up if I'm about to kill us all or get us violated. There's no point of having him in the seat if he's just going to sit there picking his nose. Be useful in the cockpit, and that means be assertive and speak up.