If attack mechanics are the yin of combat then defense mechanics are the yang; at least whenever you're engaged to things that can attack you back. The coolest thing about defense mechanics is that there are so many ways to handle the "problem" of enemy attacks but the crappiest thing about them is that they're so easy to render meaningless. In more games than I'd like to admit, you're given defensive maneuvers that you simply don't use (for any number of reasons) or you try to but they don't help you. A good defense mechanic looks good and feels good, but must ultimately be functional. Enemies are going to attack no matter what and in the interest of elegance, there should be as few ways as possible to deal with this. Having too many is confusing, having too few is frustrating, and having any at all that are meaningless are buttons wasted. Furthermore, the usefulness of the given defense mechanics are going to directly and heavily influence the difficulty level of the game.
Now let's take a look at a couple choice examples of defense mechanic usage:
Ninja Gaiden - Holding the Left Trigger/L2 button snaps the character into a defensive pose that prevents them from taking damage when struck. To be honest, I picked Ninja Gaiden somewhat arbitrarily; most modern beat 'em ups use this exact defense mechanic. The debilitating truth about blocking is that while it's just not exciting to sit there and hold a button while you get slapped with repeated enemy attacks, it works so it continues to get employed. There are certainly games which have pushed this idea a little bit in an interesting direction, but most of these instances suffer from poor execution and fail to evolve the mechanic. Now in the defense of the block mechanic, it is surprisingly difficult to break, but this doesn't make it any less disruptive. Sometimes this is intentional, as is the case with Ninja Gaiden. You cannot cancel attacks by blocking, making you very carefully consider when it's time to commit to a full fledged combo attack. The result is a highly tactical combat experience, but in addition to being very difficult, it compromises the highly kinetic combat experience that a lot of us play beat 'em ups for in the first place.
God Of War - Pushing the right analog stick in any direction quickly rolls the player character in the direction of the push. Furthermore, it cancels anything the player might be doing: attacking, moving, blocking, etc. And this applies to any attack in any combo, provided it's on the ground. This is actually one of the many reasons God Of War feels so responsive. You can react to the enemy character's carefully designed visual tells without cutting off your aggressive attack flow. The game also provides the player with a block, but not only is the block primarily designed to facilitate a completely separate mechanic, but both block and parrying (the facilitated mechanic) aren't necessary. You can play through the entire game without ever utilizing either of them. So God Of War almost wins the award for most elegant defense with its amazing rolling mechanic, but drops the ball with the perfectly good input real estate that is wasted on the block/parry. In all fairness, it would have been easy to make the block/parry meaningful by creating an enemy type that enforces it, but at that point you're contriving the simplicity of the combat.
Bayonetta - Pushing the Right Trigger/R2 Button makes the player character quickly dodge into the space just outside of the engagement zone. Like the roll mechanic in God Of War, it can and will cancel anything you're doing and immediately execute. Without directional input she slides directly backwards but otherwise she dodges in whatever direction you choose. It's exciting because it allows you to maintain your aggression in the same way the God Of War dodge does. But where Bayonetta takes it to the next level is that not only can the dodge be done in the air, but it can be performed in such a way that it will not interrupt your place in the combat flow! This is called the "dodge offset" system and when done correctly, the combo resumes exactly where it left off the second the dodge is complete. It's incredibly graceful but is also dependent on some well designed under-the-hood features; the game knows to slide the character right back into the face of the enemy when the player does this. In any case, the mechanic is brilliant and elegant and super fun and most importantly, it allows the player to continue beating the crap out of enemies until their fingers disintegrate.
Aliens Versus Predator - Traditional beat 'em ups solved the problem of defense by not actually having defense mechanics! You were responsible for positioning your character in such a way that they simply wouldn't get struck. It's ultimately the most straightforward way of protecting yourself, provided there isn't poorly designed enemy AI breaking this completely. But when done correctly, the "don't get hit" mechanic is the cleanest way of keeping the player safe because they've already taught themselves how to move. The next logical step is to develop the skill to move in creative yet cautious ways. Where Aliens Versus Predator shines is in the highly mobile attack mechanics that give the player all they need to get out of a bind or even avoid it completely in fun and attractive ways. Now in all fairness, this style of defense was made realistic by traditional beat 'em ups because they were 2d and that kind of tactical positioning was reasonable. Once the genre migrated to 3d it all changed. Not because it's any more difficult to track enemy positions in 3d games (provided you've got a camera that's doing its job), but because of a cosmetic change that occured in the move to 3d; enemy characters in 2d games could afford to simply attack to the left or right as you shuffled about. In 3d they look utterly ridiculous if they're not attacking directly at you, and having a group of enemy characters all attacking directly at you breaks the careful positioning game of yore.
As always, you should do what you think best serves the combat experience you're cultivating. :)