I use Star Trek+ as my metaphor.
A good gaming group consists of -
Kirks - Natural leaders that players that are playing their characters to the hilt and though everyone in the party may not like personally, Kirks set the pace of things that inevitably drive the game. No game session is wrong because Kirk is the center of things even if he's not doing anything. At best it's an interlude of group interest before Kirk re-asserts himself/herself to keep things on track.
Spocks - The super-competent Beta. Spocks are the advisers to the Kirks when both are in play. But they are independent enough to have their own things in the campaign going on and can easily drive things when Kirks aren't around. You can have entire subplots and sessions centering around them with little issue. Spocks are the rules guys.
Bones - Bones are the looser version of Spock. Like Spock they freely advise the Kirks and everyone else including the Spocks (who often feel they don't need it). Bones are usually less rules-harping in nature, and more high-concept. Whatever is happening in the game - they're concerned with/about it. You can have entire subplots and sessions centering around them.
Scotties - Scotties are the One-Trick Ponies whose characters are hyperspecialists to the exclusion of other things. When their specialty comes into play - they're on top of it and own that shit like no one else can, or ever will. Even the normally Rulemastery-oriented Spocks will defer to Scotties in the areas of their expertise, though they can often synergize and wreak mechanical terror on your games on occasion. You can have session that center around Scotties on occasion.
(Scotty Sub-Class) - "The Jayne" - Jaynes are the Scotty of violence. They know one thing in RPG's: combat. And that's all they give a fuck about. Jaynes, contrary to popular belief, are not as one-dimensional as most people think. You can have elaborate backgrounds, and all kinds of permutations that Jaynes will dive and drive whole game sessions around... as long as it involves COMBAT. They are usually comic-relief unabashedly so, usually in things not related to combat.
Sulus - Sulus are inexperienced but enthusiastic players learning the ropes. They may someday turn into Spocks/Bones/Kirks - but they lack the experience and end up looking like Chekovs(see below). They often find themselves getting into trouble and needing the help of the officers to bail them out. You can't have entire sessions based on Sulu's though you can have interesting campaign-twists develop out of their attempts to reach beyond their means.
Uhuras - Uhura's are the big team-player. They tend to develop into a Scotty, though it's possible for them to become Spocks as well or on occasion a Bones. Uhura's are down with whatever the team is down for. They run backup on any play made. Character depth and background is icing on the cake at best. They tend not to be strong mechanic's players, but it's not unheard of. Those that are tend to become Scotties and Spocks. Those that aren't can mutate into Bones. Many do none-of-the-above, and remain sidekicks. You can't have entire sessions based around Uhuras because like being backup, not forefront, they tend to have no agendas.
Chekovs - These are the players that invariably follow what others are doing. They don't necessarily have the strength of personality to pursue things on their own for whatever reason. Consequently, Chekovs tend to get into trouble that is obvious to other players and not themselves. They create mishaps in games due to their own self-conscious desire to not want to do anything that might make them look like a fuck-up. Their characters develop usually as consequences of what happens in the story to them, rarely out of intent. You can't have sessions based around Chekovs, and most sub-plots fail due to lack of pursuit on the Chekov's part. Chekovs never mutate into anything. They play for metagame reasons of socializing with other people who are actually there to play an RPG.