A while back I wrote a post on the inconsequential nature of most fights in D&D. Summarizing (and by all means, do read it, it’s probably the best post I have here), combat is inconsequential because its purpose is resource ablation. Originating in the dungeon crawling roots of D&D, this default approach allows the party to go through 4-6 fights a day, with tension rising as resources get expended. Only the last fight will carry the threat of killing a character or three, if things go well. Everything before was a prelude, with good tactics on the part of PCs letting them reduce the danger in that final confrontation. Troubles begin when your adventure doesn’t expect the PCs to have that many fights in a day.
This is the bit that many DMs stumble over: they consider threat of death being the purpose of combat, and therefore try to make every fight be a fight that can kill a PC. Since 4e wasn’t built for that, they have to use more and higher level monsters (or introduce drastic house rules like “halve the hit points and double the damage of monsters”, but that’s outside the scope of this post), which only serves to make combat longer. And since the PCs still refuse to go down, this results in DMs becoming even more frustrated as combat is now longer and still not doing what they want it to do.