I am a great believer in the idea that the best measure of game balance is not character power, points, stats or any such thing, but rather spotlight time. That is to say, how much time does each character (and by extension, their player) in the game get to spend doing things and having fun rather than watching other people do things and have fun. In fact, veterans of very high-power games may be familiar with a trend where the least powerful character in the game (as measured by character sheet) is the most important character in the game because their weaknesses and flaws drive play in a way that other character's awesomeness skips over it.
This behavior is something that is often taken advantage of by more sophisticated players in groups that aren't conscious of the nature of spotlight. The player creates a character that is so profoundly flawed that it ends up operating like the slowest member of a caravan - all travel must go at their pace. If it's too blatant, this can lead to grumbling, but a player who embraces this tactic tends to be adroit enough to skate the line or, if you're very lucky, use their weaknesses to draw in other players with opportunities to show off their awesome. This last is why I don't universally object to his behavior - in a bad arrangement it's toxic, but in a good arrangement it's akin to having a second GM.
Still, once a group becomes aware of the power of spotlight, it's impact is obvious, and groups often try to find ways to make sure it's shared appropriately, whether that's in the form of fair distribution, as in the case of 4e, or in the form of rotating the spotlight, as in the case of Primetime Adventures. The problem is, spotlight time is a little more fiddly than other concrete yardsticks (like XP or treasure) and figuring out how to handle it equitably usually requires some manner of artificial structure.
Most often, spotlight takes one of two forms - scenes a moments. A character can be considered to have spotlight in a scene where he has a meaningful role to play, not one where he is merely present. The wizard and thief may both be breaking into the vicar's office, but if the thief is the only one with the skills to do so, then it's a spotlight scene for him, but not the wizard. Scenes can be forcibly balanced through a variety of scene framing rules, such as letting players frame scenes one at a time. That's a little imperfect, since there's no guarantee that the framing player's character will be the only one to get spotlight. Also, a lot of gamers are uncomfortable with players having that kind of authority. As an alternative, if the GM just keeps the idea of player spotlight in mind as he frames scene, you can often end up with a fairly equitable distribution. In practice, this is little different than the usual good-GMing habit of seeing who has been sitting on their hands and drawing them into things.
Moments, on the other hand, take place within scenes. They are chances for the a character to shine, doing what they do well or otherwise creating memorable moments in the game when they did something cool. In the breaking and entering example above, it might be the thief's scene, but the wizard may have a moment of awesome when he dispels the guardian spirit that the thief disturbed. It's a problem the thief could not have easily dealt with, so it's absolutely a moment for the wizard, even if he has little to do in the rest of the scene.
In practice, moments are usually tied to the mechanics of the game, since they are usually tied to mechanical things like power uses, sine the most common moment is when the player does something that is either big (usually controlled by power systems) , or which is very appropriate yet only doable by them (and thus under the purview of niche protection). There will be moments which are a result of roleplay or player cleverness, but those are hard to plan for, and better to just accept as they come. At least on paper, a game with well designed powers and niche protection, paired with well designed adventures which challenge the range of character abilities, will have a decent distribution of moments among the players.
Thus, it's not hard to balance scenes or moments provided you're willing to commit a little effort to it. The problem is that it's very hard to do both - specifically, there's no clear way to establish an exchange rate between them. In the example given, the thief has a scene and the wizard has a moment - how disproportionately balanced are the two character? Would giving the wizard another moment square things up?
Well, yes. It probably would. 2:1's not a bad take on this, but it also leaves out one other important thing - player preference. Some players don't want scenes, and some don't want moments. A lot of times the lone wolf orphan badass is uninterested in scenes. He's willing to stand around like furniture until he's going to get a chance to kick someone's ass, at which point he will explode in fury, then go back into his box. By any abstract measure, he's getting less spotlight than anyone else, but he's REALLY happy with the spotlight he gets, and would be less happy with more.
The bottom line is that you need to pay attention to what the players at your table respond to when thinking about balancing spotlight. It's going to be a rough period of trial and error to distinguish between players who are merely unfamiliar with attention from those who actively avoid it. There's going to be a constant gravitational pull on the spotlight towards your star players because you like what they do with that, and you need to balance that impetus with the needs of the rest of the group. It can be hard, but worth it.
Alternately, you can play 4e. I mention this option because I have never seen a game solve the spotlight problem so sneakily and elegantly.
Begin from the premise than most scenes in 4e are fights. This is not strictly true numerically, but it's overwhelmingly true in terms of time spent. By making every character capable in combat, every combat scene is a spotlight scene for every character, and by making power distribution follow the same pattern between classes, moment distribution is roughly equal as well. Certainly, a given fight might throw more spotlight on one character than another, but over time, it's given to a very equitable distribution. This shared effectiveness and spotlight is a big part of why 4e, for all its faults, feels like a "fair" game.
It it perhaps curious to think of a "crunchy" game like 4e having such a well tuned system for handling character spotlight, but that just underscores the point. More than any power or rule, it is the play experience that's going to make or break your game. Balancing powers is something to be done for characters - balancing spotlight is something you do for players.
1 - A fancy way to say "Setting up a scene", including the who, what, where, when and sometimes why or how.
2 - And as extra fun, for some players, moments include things going HORRIBLY WRONG, while for others that's worse than not getting attention.
3 - and if it's not for your group, then a) Great! and b) Then you're in the same boat with the rest of us.
4 - And this, much more than any issues of nostalgia or change, is what intrigues me about the new rules in Essentials for fighters and rogues. They're upsetting this balance and trading off moments (in the form of dailies) for more general awesomeness, which will probably turn into more or fewer moments depending on player skill. That's a big risk. (Psionics, as an aside, were less of an issue for this since encounter powers were less likely to produce big moments. It happens, yes, but Dailies are still where the big money lies).
5 - Caveat - Mechanical balance is not entirely unrelated to spotlight balance. If one character is capable of doing everything and leaves other characters sitting on their thumbs, it's a problem with _both_ power and spotlight.