Actually, in v2.5, the A button does a desperation attack. There are two versions for each character, one in the air, and one on the ground. The air desperation generally does a single knock down attack. Use it when you're caught in a long combo.
Also, direction + block is a throw. Basically, when you get swarmed by enemies, but you're in a block animation, wait till the attack ends, and ram on left or right + block, and you'll toss the enemy across the screen. You can do this in multiple succession and when you're in throw animation, you're invincible until the animation finishes. The same goes for a number of other moves in each character's move lists. Basically, any move that has a "stomp" like effect has invincibility frames from beginning to end.
There is also a super jump command when you do down, then up really quickly. (Think Street Fighter 3 super jump) You can cover a lot of distance that way. This is especially useful for enemies like the grim reaper, the dragons, and the last boss.
This game is all about crowd control, which means knock downs are FAR more valuable than dishing out a lot of combo damage. (Except when you fight the last boss) Luckily, every character has knock down attacks.