By now you have probably seen (or at least heard mentioned) Mike Mearls blog post on combat balance as game balance: The Issue of Game Balance. It has generated a lot of discussion on RPGNet and theRPGSite. Here is a slightly edited post I made on the subject at theRPGSite.
Personally, I think combat balance as the be all and end all of game balance only becomes required when combats become long in real time. In early editions of D&D (up until the skills and powers books for 2e, basically), the average combat was fast. When a single combat encounter only takes 10-15 muinutes, even players who have characters who aren't strong in combat don't really have a chance to get very bored.
However, when combats start to average 20-30 minutes, such players are more likely to get bored. When combats take 45-90 minutes as 3.x combats are often reported to have regularly taken, just about any player who does not find combat the most interesting part of the game and whose character is not a combat machine is going to get bored.
To Mearls and other 4e designers, the solution was obvious: make sure all characters are equally effective in combat no matter what that does to the rest of the game. And this certainly works. No player who enjoys combat will be bored in a 4e combat no matter what character class they are playing. But the operative part here is "who enjoys combat." Combat still takes a long time and because 4e combat almost requires all players to be expert in the combat system or the group may fail, the game is much less interesting for those who do not consider combat to be the most fun thing in the game.
I would have made a different decision to solve the "getting bored during long combats" problem: make each combat much less real time-consuming than in 3.x. This would allow more combats in a session for groups who want their game to center on combat encounters (and more time for non-combat activity in a session for groups who don't want their game centered on combat), reduce the chance that the players of less combat-centered characters would become bored during combat while leaving the option open for players who just don't find combat the most exciting part of the game to play characters whose main strength is outside of combat.
Personally, I think shorter combats are one of the main things that attract people to return to or try older versions of D&D. Long detailed combats are great for those who are really into combat, but they are boring for those who find combat less fun. Making sure their characters more combat effective does not make combat more fun if combat isn't the player's thing. In fact, it can make them less interested in playing if that combat effectiveness means other players suddenly expect them to be sure their characters now pull their weight in combat and/or it if comes at the expense of the non-combat abilities that the non-combat-focused players enjoyed.