Nebulous
Legend
Personally, I have a hard time running monsters to their best ability. I have a real reluctance to really go after characters and try and kill them - I think I'm slightly afraid of what might happen if I kill someone's character.
I have this reluctance as well, but i think that a DM that really wants (wants?) to kill a PC can do it. It will probably take some planning, targeting a weakness that you know a PC has, and then trying to simultaneously hamper his allies from helping. We had one character die in the Irontooth encounter, but i have had them drop to negative several times, and don't think it would be that hard to engineer a tough fight without ending in a TPK.
I haven't even started to use coup de grace against fallen PCs, but this is a highly reasonable tactic, although players will likely hate you for it.I don't think the 4e system is at fault here quite as much as you do. Sure, PC's can get back up again - but you have to remember that monster tactics are in your hands. If you want to play smart - do so! Have the monsters fall back, pick up reinforcements, run for help, get behind cover and so on.
Also - make sure you go over the rules for healing surges, check that your players have the right amount and that they're using them correctly. Characters certainly don't have an 'infinite' number, and it may be that they're getting the rules long, which continues the problem.
"Why...exactly...did those three bugbears stab my unconscious body? There are bigger threats around!"
I do agree that there are a CRAPLOAD of healing surges, and i'm sure that sometimes people get the math wrong. That's one aspect of the game that was much easier to monitor in 1st, 2nd, 3rd edition, how much healing the characters how. Now, that's all in their hands. Sure, i can try to keep tabs on it, but the DM has plenty to worry about already.