I would vote for confidence as the most important trait. A shy unconfident dog is going to be very unhappy at an agility trial. A confident dog will do well in that kind of enviroment. Drive is more a personal taste type of thing, so pick what you like or maybe toward your upper threshold. You want to stay away from a reactive dog as they can get into problems with all the pushy or out of control dogs at agility trials. Your dog will be a pet first agility dog second. watching how a dog reacts to a sudden noise that startles it and how quickly it recovers (or if it does) can give you an indication of some things. A dog that is startled and then recovers and then goes to investigate is a dog that will be ok in a show type enviroment.
A dog that runs and won't go back or that turns and shows agression should be avoided. Agility is the most physically challenging thing for a dog (there are a few more things that are more) so good body structure is a must. Less confirmation and more field type lab seem to do better at agility. Keeping the dog a bit on the lite side helps with health issues in agility.
Light, "weedy" individuals are definitely incorrect; equally objectionable are cloddy lumbering specimens. Labrador Retrievers shall be shown in working condition well-muscled and without excess fat. Females should weigh between 55 and 70lbs and Males between 65 and 80lbs. Height females 21.5 to 23.5 inches males 22.5 to 24.5 inches at the withers.
HR Greenwoods Sealion Tsunami SH "Wave" born 3-9-2010
Greenwoods Amber Wave VCD2 RA SH AX OF WCX CGC "Amber" born 4-13-2005