Honestly? I look for a dog that I find adorable; one that I will happily live with in my home. One that fits nicely into my life outside of sports, who enjoys the things I enjoy (hiking, swimming, trail riding, and sometimes just laying on the couch watching TV). There are physical things that matter to me (medium sized erring a bit small, short coat, athletic build), but they matter as much in regards to my life as in regards to sports.
I'm pretty heavily into agility. I have two dogs right now, one who has earned her retirement and will be allowed to do so as soon as she tells me she wants it; the other a baby dog who is just starting his competition career. I work with the baby dog on agility most days (weekly class, online class with regular homework, club practice weekly, trials). Even with all that, I'd be shocked if I spend six hours a week on agility (not counting trial days). There 162 other hours in the week where I have to live with my dogs. A dog who is phenomenal for those six hours isn't worth it if I don't love living with them the other 162.
If I woke up tomorrow and couldn't run agility any more with either dog, they would still be the most wonderful partners for me in whatever else we chose to do - even if it was just as pet dogs.
Although I don't think I'll ever get a black agility dog again. Holy cow is Gusto hard to keep cool.