An ambling or "pacelike" walk has an impure, irregular rhythm because the interval between the hind foot and the same-side fore foot is shorter than the interval between the forefoot and its diagonal hind foot. (You rarely see a clear pace where the same-side feet move in perfect unison.) Because the rhythm is wrong, and rhythm is the most important quality in riding, this is a big problem.
Some horses are born with this faulty gait but others can be ridden into it when they are asked to walk on the bit too early in their training. In either case, an ambling walk is a serious deterioration of the gait and is very difficult to fix. I would not recommend buying an ambling horse in hopes of repairing the walk, no matter how kind its personality or lovely its trot and canter.
A trot can become irregular if your horse's forefoot touches the ground before its diagonal hind foot. (It can be desirable when the hind foot touches the ground before the diagonal forefoot--a dissociation known as "diagonal advanced placement" or "DAP.") An irregular trot is fairly easy to improve by riding your horse forward and strengthening him. In fact, the trot is probably the easiest gait to improve because there's really not much that can go wrong with its rhythm.
At the canter, some horses can be "lateral" or "almost lateral," an impurity that arises when the inside hind and inside fore touch the ground at almost the same time. Other horses can be lazy and what we call "four-beat-y", when the outside fore touches the ground before the inside hind. Now, we all talk about a four-beat canter, but you hardly ever see one. What appears to be four beats is a lack of jump due to a high degree of tension in the horse's back and very little suspension.
What would I do with a lateral canter? In the first place, I wouldn't buy the horse, but if I already owned him, my first instinct would be to ride him in a little bit of an overtempo to develop more jump and air time. If that didn't work, and the horse were trained enough, I'd try a little shoulder-fore, shallow half-pass or haunches-in, maybe even on a circle. If he were young and green, I'd try small jumps like cavalletti-height in-and-outs.
Learn to recognize the different "types" of gaits in Jan's article "A Guide to the Gaits" in the September 2008 issue of Practical Horseman.