Ok for an example. This is straight from the IGN review.
" For example, one of the newest systems in Street Fighter IV is the Focus system, which allows a character to charge up an attack that will knock an opponent down if it connects properly. For a beginner, the Focus Attack isn't necessarily needed to win. He or she could simply rely on the traditional assortment of fireballs and dragon punches to get by and have a good time doing it. But if the Focus Attack seems appealing, performing it is a piece of cake -- just hold down both Medium Punch and Medium Kick together and charge it up (or alternatively, just assign the attack to a button of your choice in the options menu). With a simple input, the player now has access to an attack that adds another layer to the combat.
But that's not all there is to the Focus system. Players who want to dig even deeper into the technical elements of it all will learn that the Focus Attack can also absorb a single oncoming strike without being interrupted. Although you take damage for the absorption, that damage will heal given time. In this way, the Focus Attack becomes a defensive technique as well as an offensive one and offers even more strategy for players to consider.
And there's even more. You can also cancel a Focus Attack mid-charge by dashing out of it, which can help you play tricks on your opponent. Or, you can expend a portion of your Super meter (which I'll touch on shortly) to cancel a Special Move directly into a Focus Attack. This single system depicts the broader Street Fighter IV picture: it's so easy to pick up and play but there's enough depth to keep even the most hardcore competitors satisfied.
There are a number of other systems in Street Fighter IV that players can keep track of. There are two meters that can be used during a match: the Super meter and the Ultra meter. The Super meter fills as you land attacks and fight normally, while the Ultra meter (or Revenge gauge) builds up as you take damage. You can use the Super meter to execute Super Combos, which are elaborate attacks that can be chained together with normal techniques for some spectacular combinations. On the other hand, you can use the Ultra meter to perform Ultra Combos, which are more cinematic, devastating attacks that can turn the tide of battle in your favor.
Once again, these systems can be enjoyed on a number of levels. You can -- for the most part -- ignore them if you wish, or you can just wait for your meters to be filled and then attempt a Super/Ultra Combo. But if you're interested in even more technical applications, you can use a quarter of your Super meter to perform Ex versions of Special Moves, which generally strike in different ways than the normal versions."
This is an example of a deep complex sub system that permeates SFIV. This doesn't even touch on everything, just the big things.
Now beyond this each individual character is going to have their own meta-game within this subsystem. This doesn't even begin to count all of the spacing/frame counting/timing/exploits that pop up after a game has been out for a while. This creates innumerable possibilities, combinations, strategies, tactics, and methods.
Brawl doesn't have anything remotely close to that in terms of scale of depth. Melee sort of did through physics exploits, and a few built in things, but even that was shallow compared to most fighters and then brawl went and removed the sub systems (like L canceling, or the fundamental ability to combo) that existed in Melee in order to simplify the game for a wider audience.
Brawl has some depth, competitive players will always find depth. But that doesn't make it a deep game. It just means that there is some degree of separation between a ten year old girl playing Yoshi 'cause he's cute and the guy who plays for twenty hours a week and hangs out on the smash world forums.
