I skimmed through and took a VERY informal counting, and took MANY liberties and assumptions.
But here is a rough tally on reasons that PCs have died, as told in this thread so far.
13 negative bloodied damage (including taking recurring damage from an effect or just hit really really hard when already low)
10 overwhelmed/outmatched (resulting in either a tpk or abandoning one or more pcs to save the others)
18 failed death saves (and the rest of the party is nearby but unable to assist for some reason/dazed/isolated)
4 Misc other (became bait to be killed so others could ambush, forgetting/misunderstanding rules/etc)
Again, this is just a very very imprecise counting. But being cut off from the party while you bleed to death (they can't reach you in time, or don't have line of sight/effect, or they're dazed/occupied, or don't have the movement to get to you, etc) seems to be the winner...
while negative bloodied is the silver medal.
of course, many people also talked about near-deaths but in the end they were saved and therefore not counted... so this informal count won't help judge the general lethality, merely give some idea as to causes of death in 4e parties that shared their experiences in this thread.

But here is a rough tally on reasons that PCs have died, as told in this thread so far.
13 negative bloodied damage (including taking recurring damage from an effect or just hit really really hard when already low)
10 overwhelmed/outmatched (resulting in either a tpk or abandoning one or more pcs to save the others)
18 failed death saves (and the rest of the party is nearby but unable to assist for some reason/dazed/isolated)
4 Misc other (became bait to be killed so others could ambush, forgetting/misunderstanding rules/etc)
Again, this is just a very very imprecise counting. But being cut off from the party while you bleed to death (they can't reach you in time, or don't have line of sight/effect, or they're dazed/occupied, or don't have the movement to get to you, etc) seems to be the winner...
while negative bloodied is the silver medal.
of course, many people also talked about near-deaths but in the end they were saved and therefore not counted... so this informal count won't help judge the general lethality, merely give some idea as to causes of death in 4e parties that shared their experiences in this thread.
