One of the reasons, I think, is that I love to employ varied terrain and hazards. If the battlefield stays interesting, there are always options outside of just at-will repetition. The advice here to use the monsters to keep the combat moving is excellent. Its hard to say its a boring fight, even in the 15th round, if people are still moving and shifting all over the place trying to fight for control of strategic areas or force choke points or whatever.
I think this thinking is huge. Mearls wrote an article about it which I only saw turned into fodder for "4E is the Suxorz" threads, but it changed up how I ran the White Dragon in Kobold Hall, and it was awesome.
One thing I've experimented with (so far not to total satisfaction) was having an index card with the descriptive (not necessarily mechanical) effects of special terrain in the big encounters. The Leader among the PCs gets a Perception roll, and if they pass it, they get that card handed to them. So they know a bunch of potentially big tactical information, but they have to get that info to the other PCs...kinda what a Leader should be doing, no?
Again, I haven't done it enough or had it work out as well as I'd hoped, so there's some room for improvement. Instead of going with the Leader getting the check, make the check relevant to the area: say, if it's a woodland battle, the Ranger gets to roll Nature or something. Maybe make a separate index card for easy-to-notice terrain effects, one for moderate and one for hard, and turn into a skill challenge that opens up the combat.
Something like that. Anyway, the idea being that it not only forces me as the DM to make interesting terrain effects, but also gives an easily prepped way of getting that info the PCs in a fashion that might even promote roleplaying, as opposed to just saying "Hit this stalactite and it'll fall, causing XdX damage to anyone in that square."