I think certain factors have a huge impact on the length of combat:
1) The DM - the DM generally has more to do than all the PC's (keeping track of each monster, selecting their powers, rolling dice for each monster, etc.). If the DM is slow, the combat is going to be slow no matter what the players do. (Sometimes you can't help it - if some of the players are more tactically gifted than you, then you need to spend extra time choosing your monsters actions to challenge them.)
2) The Players - some people are just slower to decide their actions than others. Not much you can do about that. Some people are ineffecient when resolving their turn even when they know what they want to do. There are various tricks to speed up the players turns that have already been discussed.
3) The Characters - some classes and powers are much simpler to play than others. A warlock that blasts a single target is faster than a wizard who Fire Shrouds and has to roll attacks against 7 different opponents.
4) The Build - some powers and feat combo's are simply better than others, and if a player builds a character to take advantage of this, then they tend to do so much damage (or help other people do so much damage) that combats tend to finish within a 2-3 rounds. (A lvl 6 tempest fighter built to optimize Rain of Blows can have a +17 attack bonus with CA, and be doing up to 4d8+44 damage to a single target, or 2d8+22 to two different targets for example - elites and brutes tend to die quickly, especially when action points, crits, and buffs from other characters are factored in.)
5) Teamwork - if the players know how to make their characters work together, they hit more often and do more damage when they hit (especially if warlords and battle clerics are involved0).
If you have a party of 5 players who don't waste time, have damage optimized characters, use the most effective powers and feats, and know how to build characters that synergize with each other, combined with a DM who knows the rules and doesn't spend to much time deciding each monters actions, the combats will be an order of magnitude faster than a group that has less optimized builds and discusses every potential action for a couple of minutes on each characters turn.
I'm not saying one way is better than the other, just that one will result in shorter combats than the other. The point of the game is to have fun, and if you spend 2 hours on a combat but enjoyed it because you were all talking, joking, and roleplaying for 5 minutes on each characters turn then you did it right.