Mal... I agree that the "personality factor" is a big thing. Group dynamics are one of the biggies for marking the success/failure of any trip. So... one consideration is the SIZE of the group you are traveling with. If you have a group of 3 looking for a 4th it's alot different than a single looking for a 2nd. I mean, if you have other people you are use to tripping with, it's less risky to bring along an unfamiliar face. If that new member isn't up to the standards of the rest of the group it's alot easier for the group to "absorb" the problem.... You can alternate paddling/fishing partners, and there are more alternatives for sharing camp chores, etc. When it's a twosome, you're alot more likely to get in each others face. LOL.  ::)
Yellowbird also brings up a good point about reading past posts. If someone makes a post that catches your eye, you can look back at their other posts and get a pretty good idea of who you're dealing with.
I just caught the number of members on qj.... over 450. Wow. I would hazard a guess that half of that number will be tripping with someone new in the foreseeable future, either by choice or by necessity. Maybe half of those will be looking for a partner with experience in the Q or BW. Well.... that's over 100 people. I suppose it would be overly optimistic to expect that more than 1 in 10 of those would be on the same sheet of music in the important areas. But... that still leaves a healthy number of folks to get to know a little better.
;)