PC durability is one of my favorite things about 4E. It gives you a chance to determine the style of how you want to play. If a DM is willing to pull punches and only punish outright stupidity, death can be fairly rare. If the group wants to go old school however, there's plenty of ways to make a party feel the pain. We like things a little more old school, so I DM with the gloves off, and we see PC death on a fairly regular basis, usually every two to four gaming session.
The most common cause is the fact that we have 8 PCs, that leaves me a lot of monsters to focus fire on a squishy when I get the chance. Not to many PCs can survive for long when 8 bad guys are taking turns pounding on them.
Poor tactics is another frequent cause. We're thinking of renaming "Come and Get It" to "the Tank Dies Again," because our great weapon fighter sometimes gets a little overzealous in how much fight he wants to pick, and a number of PCs hate a particular gnome wizard who gets a little too happy with his area of effect dailies.
As a DM, the simple fact that I'm willing to be mean every once and a while increases the game's lethality. The occasional coup de grace on an unconscious PC has ensured my players never leave someone on the ground making death saves, and clever use of terrain to split the party has allowed me to separate the party and then gank a PC out of reach of the others.
Last, the simple fact that we strictly enforce the only three death saves a day rule, and that I don't allow a five minute adventuring day ensures that any end of the day encounter will always feel pretty dangerous to at least one PC. This one's important, I honestly don't see how more PCs don't die from this rule in other people's games. It's claimed a lot of PCs in mine.
All in all though, I think 4E got it about right. Single PC death can be fairly easy to achieve, but a TPK usually only happens due to poor encounter design, horrible luck, or poor PC tactics. We only came close once, and the group was able to survive by stuffing bodies in a haversack and fleeing as the defenders held a choke point. That fight definitely had an old school feel to it.