If horse do not have straight legs, when it's moving around they will not be able to absorb properly and this might lead to different kind of injuries or during hard work like jumping or cross country it might cause soft tissue damage.
Also you should make sure horse is standing straight and isn't struggling in keeping their weight on all legs also should be able to pick all four feet up. If horse is refusing in picking up their leg it might be because it's in some kind of pain or they're using this leg to take of some weight of some other legs that is hurting. It can also means it's simply not behaving but it's your job to figure out why horse is doing something or refusing.
Tried to be as detailed as I could, hope I helped :))
You have to look at the whole picture and see what is its confirmation like. You look at horse's proportions and if u generally like it.
Then you look at the details from head to tail.
Has it got big enough nostrils to breath during hard work?
Mouth big enough to fit a double bridle (sometimes when you're competing it's required to ride in it)?
Can you fit your fist in horses throat latch (if not the horse will have a difficulty in bending their neck and breathing)?
Is neck proportionally long to the back?
Has it got nicely defined wither?
As you stand in front of the horse is its chest wide enough to hold the lungs?
Are horses legs good columns of support (straight and strong?)?
Are horses feet straight or maybe pigeon toed or opposite?
Is horse's back appropriate length (should be able to fit one hand between the last rib and horses pelvis)? If horse will be long backed it might have difficulty in stretching down, bending and keeping balance, can be hollowed as well and difficulty during jumping. If horse is shortbacked it will overreach and might be unbalanced.
Is its back side nice and round and big enough to encourage the horse to use them (if not horse will be constantly on its forehand and have difficulty in using hind quarters)
Now stand behind the horse and see if the horse's legs are good columns of support or maybe cow hoxed or opposite?