Almost everyone who trims trees gets at least some of these things wrong. All these issues are discussed in detail in my other posts, so I'm only going to touch briefly on these items here.
In order, these are the most common errors I see in almost every managed tree:
Too many branches originating from the same point. Branches should be staggered. Bonsai artists are usually exceptionally diligent about this, and everyone else can learn a lesson from their styling. Unfortunately, if you don't avoid them very early, you're stuck with them for good and have to make the best of it.
OVERLY-LONG, DISPROPORTIONATE, UNTAPERING, TOO-STRAIGHT BRANCHES
These branches overwhelm the tree's balance, especially in relation to the trunk, create weakness, and just don't look good. This is equally caused by over- AND under-pruning, ironically usually simultaneously.
WAITING TOO LONG
Once you play catch-up, it's a long recovery that may never turn out right.
Cutting out way too many secondary/intermediate branches, leaving almost all growth at the very fringes. Good thinning does not remove everything below the top. Ramification throughout the tree is important.
A tree will always look pruned (not in a good way) if you cut the tips off the branches, pretty much anything larger than the thickness of a pencil. This also leads to over-thinning the canopy if you're thinning out the ramified tips too much. Sometimes, however, this is necessary and good plans will correct this remedy within short order.
VERTICAL BRANCHES/WATER SPROUTS
These are hard to avoid after major pruning in some trees and usually look bad and are unhealthy, so diligent management is key. Frequent, minor pruning is much better. Pruning more than about 25% in most trees is best avoided, and with good, frequent pruning, more than 10% should rarely be needed.
LOW CROTCH ANGLES
Branches should exit from the trunk at angles that increase strength and look good. Included bark is best avoided.
This is the queen mother of all pruning mistakes, along with its little brothers, pollarding and Crape Murder. Luckily, most people realize topping is just a terrible thing to do in almost all circumstances. Unfortunately, most people are guilty of pollarding to a point, whether unintentional or semi-intentional.
Ok. Pay attention to this in your own trees, learn about and check out what a really good tree looks like, and be careful who attacks your trees. Pruning is, after all, an attack from which it must recover. You can assault it with infrequent bloody violence or give it a frequent attack of sweet, sweet love. You decide.