Corporal, a good walk will have a very clear 4 beat - a bad walk, will become lateral or similar, and often can change to a 'shuffle' and become nearly 2 beat.
In a good, strong, correct walk, the horse will be marching actively forward, and the rider should feel the topline stretching and contracting. If the horse is behind the leg or has a poor walk, often the head will bob up and down rather than forward and back.
You should be able to feel when each hoof hits the ground. This is part of being a good rider, until you develop a sense of where each hoof is at any given moment, you will not be able to positively influence the horse's way of going. In walk, you need to be able to count '1-2-3-4' as each hoof hits the ground. Watching from the ground, you should be able to very clearly distinguish these 4 beats, they should not be 'kind of 4 beat'.
It gets harder to maintain this clear 4 beat walk as we start demanding more collection from a horse. Many horses will start to get tense, and tension leads to problems in the walk. That is why I try to do as little work adjusting the walk as possible, and most of my walk work is done out on a trail or on roads as the horse is more inclined to walk freely forward and maintain that lovely clear 4 beat.
I will have to upload a video of my thoroughbred walking. He's just come back into work after an 18month injury spell, but his walk is of such good quality that he would walk for at least an 8 and more likely a 9 in a dressage test with more work on his 'buttons'.
He is very free through his back, swings his hind legs right under his centre of gravity and overtracks well. You can set a metronome to the beats in his walk. That is how a walk should be!