The current crop of D&D tends to balance the classes with a focus on equality in combat.
Such that for any given level and any pair of classes, they do roughly the same amount of damage within a 6 round combat
Loosely put, nobody sucks at any level during combat compared to another PC. Since most of D&D is combat, that's the balancing point.
In prior editions, having less combat skill meant you had greater skill in other areas for other enounter types. Thus it was justifiable for the Bard to suck during combat, because he could do cool things outside of combat.
Compare this to 1e. A first level wizard is so worthless, that if you were recruiting for a 1st level mission, you'd be better off taking a 1st level fighter.
Later, at 15th level, you'd be better off taking the wizard than the fighter.
While the 1e designers saw this as a long term balance (life is harder at low level for a wizard, in trade for more power at high level), it didn't necessarily make for fair and fun game play on a per session basis.
Sure 1e had balance goals in mind. But they were 1970s idea of balance.
The 21st century idea of balance is that each PC in the current game session is able to contribute and add value in a fairly equal fashion. If there is a class that a min/maxer would say "only an idiot would pick that class, it sucks" or "I always play this class, its the best" then that class is out of balance.
Why does this form of balance matter?
For a GM, it matters because it makes measuring encounter difficulty a more consistent process. Each PC is balanced, so they are interchangeable, to an extent. This in turn means all you need to know is how many levels there are. Sure, the GM could tweak set up special encounters to challenge or spotlight a class, but on the average, it's all the same.
For a player, balance matters because nobody likes their character concept to suck so badly that they feel like the tagalong sidekick to the heroes. Sure the guy with the uber-class is happy with the imbalance. Deep down, nobody else is.
In 4e, it seems like the focus is combat functionality. Some classes may do better in direct combat, but all the classes have features to make sure they are actively participating in a fun and useful fashion. Thus, the guy playing the "least fighty" class can still feel like he kicked butt, because he actively contributed in a meaningul way.
That's the new balance.