I've been thinking about this topic for a while, since Wrecan's poll on encounter length. Like 4e itself, the poll assumed a party of 5 heroes facing a group of 5 equal level monsters. But is this a good balance? Should that be the default count?
Previously on D&D
Earlier edition were much more fluid when it came to encounter balance. A fight might be a single orc or it might be a dozen orcs. Fights were not balanced, but worked as well with a half-dozen opponents as two-dozen as a single foe. 3e tried to add a dash of balance to encounters, with the system being based around a single high CR threat to a party of 4 PCs but with sub-rules for one or two foes. As the edition aged, it was noticed that many DMs were making higher EL fights with multiple opponents, which made for more interesting tactical fights.
When they were designing 4e they used multiple opponent fights as the baseline, to make for tactically interesting combats. Multiple opponents make for interesting dynamic fights, giving every character something to do and an opponent to face.
Types of Fights
It's hard to decide what should be the baseline of the game, as different fights have different needs. Each does something different, scratches a different itch.
First, there are mook fights: incidental battles with henchmen and thugs. In action movie terms, these are where the heroes defeat a couple waves of bad guys with one or two strikes or an entire room full of toughs. They're quick, and often done more for story reasons than tactical combats. There is little danger of death, and success is determined through expending as few resources as possible.
The opposite of mook fights are boss fights. These should be big tactical affairs with interesting terrain and unique elements. Boss fights should take some time. When set-up right, 4e is excellent at these. Combat length is a feature here, not a bug. These can be tricky as solo monsters have been handled poorly, frequently being big bags of hit points that are little threat with the resources a party of heroes can muster.
There's also sub-boss fights. Mini-bosses. 4e is excellent for these fights, as elites are tailor made for mini-bosses or bodyguards for mini-bosses. These should also be longer fights so again combat length becomes a benefit.
Then we get to the baseline: fights against an equal number of potent bad guys with the possibility of death or injury. These are a little harder to place in the narrative. They're not throw-away encounters but neither are they boss fights. They're big tactical affairs because the Game wants then to be, rather than out of a strong narrative need. But once you get into the encounter design space of 4e, these can be a heck of a lot of fun and it's where the edition really shines.
Honestly, this is one of the places I feel 4e could have been done differently. When planning my games I often had to work to accommodate this kind of fight, the game ended-up dictating the story more often than not. There was an initial learning curve where I had to learn to shape the story to lead to group fights; there couldn't be a single monster — even when the desired creature was antisocial, such as a bullette or owlbear — as fights required 3-6 opponents. It was easy enough to manage, but for my first couple sessions I struggled to tell the stories i wanted to tell.
Working With Mooks
4e has problems with one of the above types of combat: incidental encounters. These are small combats that provide a quick action break in the narrative. This might be a quick mugging, distracting some guards, the villain's disposable thugs, or bar fight. Anytime where it makes sense for there to be opposition, but not foes comparable to a party of heroes. Or, especially, when the PCs start something or walk into an un-planned fight. These are fights where there should't be a danger of serious injury. Instead, these fights might threaten slow down or delay the heroes, or just offer the players a quick break. But devoting a solid hour of play to a mook fight is a little much.
More often than not I take the easy way out when it comes to mook fights, and just make them regular monsters. This does a disservice to the players and the story as these fights are not that interesting, and the time spent making them mechanically or tactically interesting could be better spent with other battles. Combat for combat's sake is, well, lame. Homegames are not Living Forgotten Realms, there is no mandatory number of fights per game.
It's easy enough to employ any of the combat accelerating tricks for mook fights: doubling damage and halving hit points, limiting PCs to At-Wills, or only using lower level monsters and minions. A big vulnerability also works, tweaking the bad guys to be susceptible to the PCs' attacks or an environmental hazard. 4e zombies offer another option, as they "die" when critically hit, which is fun and this trait could be added to standard monsters to make them easier to dispatch... if the heroes are lucky.
Minions can die a little too quick to rely entirely on them, but adding extra hit points is awkward. When using only minions, I favour giving them a saving throw to negate the first damage taken, or a save against any damage. This makes killing them easy but makes for simple bookkeeping.
It's also possible to use regular monsters and have them flee or surrender when bloodied. This requires some forethought, as many monsters get tougher when bloodied, so those shouldn't be used. A good ol' Dave "the Game" Chalker Combat Out also works. But, since mook fights are simple affairs these should not be big complicated puzzles or overly thought intensive. Challenging or investigative Outs are better for mini-boss fights or as a mid-fight change of pace for long multi-phase boss fights; something simple like killing the lead monster causing the rest to surrender works very well.
I've also seen Skill Challenges used in place of small combats. This works and scratches the itch to roll dice, but isn't as satisfying as using spells and all the things that make you character class fun. Controllers don't control, defenders don't defend, and no one deals damage.
Thinking about the inevitable 5th Edition, the baseline of combat needs to be considered, if there should even be a single baseline for combat.
I regularly push for flexibility, and combat is no different. The game should easily accommodate multiple play styles, including variations on combat intensity. The game should accommodate many quick mook fights, several intense group fights, or a couple infrequent boss fights. Encounters should not be standardized, because each encounter should be different.
Multiple types of monsters might be the way to go. Or quick templates or formulas that can be used to turn standard monsters into simpler or harder threats. 4e seems to have forgotten templates, not because they're not an excellent mechanic, but because they're harder to make well and most were poorly designed (unsurprisingly, given how early in the edition they were written). Instead of just being able to make monsters tougher or customize by adding powers, it should be possible to simplify monsters or weaken them. It's simple enough to house rule this, but the game should support and aid more mechanical customization.