But I think most important of all, what makes a good agility dog is a dedicated agility student/handler.
^^^This!! excellent response.
Most people get in agility with the dog they already have. When choicing a dog they need to consider how they want to run agility.
Ask your self a couple of things.
Do you like adrenaline rushes?
Do you get easily frustrated?
Because both of those things often happen when running and training a high drive and fast dog.
Are you prepared to train longer (as in months/years) to develope the higher skill sets that both you and that dog will require?
Some people like the steady eddies while others want the rush, no matter what the cost is lol.
Oh yeah, and make sure you have available very good trainers to train with and being prepared to spend for those trainers. There are all kinds of trainers out there but they don't have the skills or the experience to teach you how to run a fast dog.
A lessor trainer can squeak by with low drive and moderate to slow dogs.........
Angulation too straight= possible shorter career, higher impact, shorter stride, vet bills are also possible, horrible jumping style which can result in dropped bars. etc.
Too over angulated: the more angulation to more unstable the legs are especially the hind end and they also tend to have less than ideal jumping styles etc.
Body type is also important: too heavy=hard on the dog, has to jump lower
But having said all dogs of all walks of life compete in agility and do well.