Last weekend with HotDQ, our 5th level moon druid died. And my 5th level Shadow monk went down to 1 hp.
At TotalCon this weekend I had half the party down at one point, but given it usually takes 4 or 5 rounds to bleed out, no one actually bought it. Between stabilizing and healing, characters were able to avoid death. I do like that PCs can and do go down, though -- it means the characters that are still up have to decide whether stabilizing a buddy or continuing to hammer the bad guy is better, and meaningful decisions are the heart of what makes D&D fun.