Here's an example.
Recently, the trailer for the live action Gatchaman movie has been making the rounds. We know it here in America as Battle for the Planets. Five people, each with their own vehicle and special weapon, fight off an invasion from another planet. Each episode features an attack by a giant monster stomping on a city someplace, and G-Force has to go out and fight it. Usually by combining their efforts as a team to transform their spaceship into a giant phoenix. Don't ask. I don't understand that part, either.
However, it would make the perfect roleplaying game.
Each of the five characters has his or her own schtick. Mark wears the eagle uniform, has a razor boomerang, and flies the jet. Princess wears the swan uniform, uses the yo-yo bombs, and drives the motorcycle. Keyop wears the swallow costume, throws an exploding bolo, and gets the weird tank (which the wiki calls a dune buggy -- whatever). He also babbles incoherently, because in the original Japanese he had a foul mouth and so he was dubbed speaking nonsense.
Mark never drives the motorcycle. Keyop never uses the razor boomerang. Each character has his or her own schtick. That thing that makes them special, unique, and distinctive. Perfect for a roleplaying game, and the place where some games fall down, in my opinion.
In an effort to give players maximum flexibility and the most game options, some games don't focus on sharpening those distinctions. We don't want to limit player actions, do we?! Well, yes. We do. It's a game, first of all. Everyone wants to feel special. Everyone wants to have their schtick. If the thief can be as good as the fighter in fighting, well then the fighter is no longer special. If the street samurai can hack a network almost as well as the netrunner, then what's the netrunner really there for?
Sure, granted, its more realistic. After all, everyone is a collection of their experiences. I, for example, am pretty handy with a gun, in addition to being able to write. I'm also pretty good at fixing things. So if I were going to stat myself out in GURPS or Savage Worlds, I'd make sure to give myself the writing, gun, and repair skill. That's realistic.
But we're playing a game. What kind of game? A roleplaying game. So by giving everyone the ability to create almost any kind of character, you take away the distinctiveness for each character role. By giving players the ability to create anything, you end up giving them nothing. Each character should have its schtick that it does better than anybody else.
There are a host of things you could do to reinforce this idea. Maybe, you pick a class, and then anything not in that class is more expensive. Maybe, you don't even allow one class the ability to pick something from another class. Or maybe, you just state outright that the players should decide on a specific archetype and try to stick to it (in other words, there aren't any rules barring you from picking something outside your class, but you explain that that's not kosher).
What is special about our type of game -- roleplaying games -- is that we get to pretend to be someone we're not. It's "let's pretend" with a form. We all see ourselves as Harry Dresden, Elric of Melnibone, or Paul Atreides while we're reading the book. This hobby allows us to do that, and tell our own stories (even if we're not specifically playing Harry Dresden, we can play a Harry-style character in Merry Old England...). In that case, a good game has a strong sense of itself, and presents only those elements and options that serve that vision. Rather than the kitchen sink, a game should set limits that service the paradigm.