Structure limits the dog, or enables the dog. I could go on about how each fault affects the dog's motion, but that would be a heck of a long post, and I'll stuff it for now. Maybe I'll prepare a seminar on it some time. The biggest thing to keep in mind is that if your dog is not in pain, or caused pain, by doing what you're doing, there's absolutely no harm in it. I do hate when evaluators tell owners "your dog is put together so badly you shouldn't do agility!". If the dog isn't hurting, and it's willing, who are they to say no?
Structural talk aside, here's a lovely example of a young dog. I like how his rear is nicely angled, but without having that overangulated look that some BC's have. His butt is hairy, so keep that in mind when looking at the angles. His shoulder is quite nice for a BC, and much better laid back than many. Nice neck, nice topline. Only thing he does wrong right now is that he toes out a bit - but he is not even 11 months old right now, and I am absolutely sure he is not done filling out yet, so that will resolve. And if it doesn't, that is hardly a major fault!
Here's Mr. Gorgeous!
All Zet Air Attack Strafe at 10 months, 3 weeks old.