I wrote on this subject before, so I'm just copy and pasting. =D
Character Selection - Counter Picking
Stage Selection - Avoidance of traps and gimmicks
The First Stock - Setting the Tempo with an early lead.
Edge Guarding - Denying your opponent the ability to get back on stage.
Recovering - The ability to return to the stage without incurring damage.
Punishment/Risk - Dishing out damage while being safe.
Taking a Stock - Either through brute force or a ‘gimp’.
Space Control - The ability to deny area.
Neutrality+ - The ability to return to a neutral or positive posture.
Defense - Maintaining the lead.
Offense - Building damage through the recognizing of strings.
That's Brawl in a nutshell as a game. You can learn how to succeed at those facets of the game. Character selection no doubt skews the learning process. There are substantial learning curves in the game when you compare the various tiers of players to each other. Essentially how characters perform at the various areas of the game will dictate the level of success that they are going to see in tournament and personal play. Brawl isn't an overtly difficult game to get the basics of your character of choice down. It doesn't take more than a few minutes to realize that Snake's Tilts are -really- good, and that you want to recover as high as possible with him; more so, it is fairly evident that the best way to approach with numerous characters is with their forward airs, and so forth. Truthfully, when it comes to the schism of how the game is played by novices and pros, it is a matter of fundamentals.
The game drastically changes when you consistently power shield attacks, consistently recover at the optimal rate for each character, and punishing attacks. Suddenly overpowering strategies such as projectile spam no longer are relevant, aside from biding time. Due to these differences, a novice player will hold Ike in high regard due to his ability to dish out kills easily. Unfortunately for Ike, there is a large window of punishment after one of his moves, yet most players are incapable of finding the block button. Due to this, Ike becomes a dramatically better character choice in novice play. The same could be said down the list, Falco becomes a much better character when you are properly using his chain grab to spike, buffered dacus attacks, lagless lasers, and so forth. Due to these differences in play, it's hard for novice players to accept tier lists as having merit in their normal play, or they flat out refuse to accept them as a reality.
Chiefly in novice play, players are able to fight each other on just the merits of their individual understanding of the fundamentals of the game. Simply being able to rely on the defensive options inherent in all characters [though some are more blessed than others: Falco's amazing spot dodge] is enough at that level. The last time I went to an anime convention, I actually played every match with nothing but Mario. I three stocked essentially everyone there, but more importantly due to how well I was succeeding at the game I had many players claiming that I was using a top tier character in the game. To them, it felt that Mario was the best because I was a vastly better player. Perception is key, but they lacked the understanding as to why I was performing better than them which was a disparity in individual skill level.
Once players begin to understand the fundamentals of their characters, the tier list becomes actively important for your individual success. The moment the Falco player in your group of friends begins to use a Chain grab against you, your character selection choices drop due to how incredibly powerful the move is. Literally a third of the cast is removed as a viable option, and the bulk of the others yield 40% or greater damage a stock to Falco. Let's say that you have enjoyed playing Link up until this point with your friends. Unfortunately for Link, Falco is able to chain grab him quiet easily, and put him into a position where one grab can honestly lead to death off the side of the stage. Suddenly your enjoyment level and perceived skill drops tremendously in your friend group. It doesn't take a rocket scientist to walk forwards and do down-throws, which is why I'm using Falco as an example. In fact, the 'grab characters' [Dedede, Falco, Pikachu, Ice Climbers, and Olimar] shape the tier list more than most characters. It isn't hard to get a grab in Brawl and due to that, these characters dramatically change the landscape of the competitive field due to how they are ability to shutdown characters completely. Most characters when you look at the Tier list and wonder why they are positioned low can simply be answered by looking at the match ups that they have against the grab based characters.
Long story short, if no one branches out beyond the basic fundamentals, tiers aren't too important. It is a matter of the mastery of the fundamentals, but once you expand to learning your character and become aware of the options of your opponent everything changes.
