Ongoing damage can be a culprit, but I think the vast majority of our deaths are from failing too many death ticks. Healers run out of healing, second winds are used up, and getting over to stabilize the dying just isn't always possible, with monsters pushing, pulling, slowing, and immobilizing people.
Yeah, the Needlefang Drake swarm one was rough. Uninformed, any low level PC could walk into that one and die. But, part of it is that the players should ask for various Monster checks like Dungeoneering so that they can find out some basic actions to NOT do. In the case of the swarm, they have a deadly swarm aura, so don't stand next to them both at the same time.
Another thing to do is to keep the Leaders in the middle of the group as often as possible and to NOT move more than 10 squares away from them. They need to be able to get within 5 squares for a heal power or all of the way there with the Heal skill within a round or two.
And a major part of the Leader role is to heal. I suspect that some players in campaigns where a lot of deaths occur can partially blame it on the players of the Leaders. The moment a PC goes down, a Leader and possibly another PC should be moving to heal/protect the downed PC. Instead, I suspect that some players think "No problem, I have 2+ rounds before I have to heal him" and then the Leader gets cut off from the PC or gets a condition put on him and a few bad dice rolls later, the PC is dead. And, it's not just Leaders. Any PC can try to stabilize an unconscious PC when the Leader is out of heals or busy and far away.
In fact, Leaders should often (but not always) be healing the moment certain PCs (especially squishies) get bloodied.
And it is also the responsibility of a player to have his PC take his Second Wind. I've noticed that players in my group tend to almost never do this because the heal powers are so much stronger. Yup, it's a standard action. Yup, healing powers are better. Deal with it. Not every round requires an attack roll. In fact, many rounds with an attack roll miss anyway. Heal, then attack.
In synopsis:
1) Monster checks are your friend.
2) Maintain mobility for Leaders.
3) Almost always heal or at least stabilize unconscious PCs immediately.
4) Often heal bloodied PCs immediately.
5) Use Second Wind. Saving resources can lead to death as often as saving from it.
4E is group oriented, so the group has to assist seriously damaged PCs in order for everyone to survive. That sometimes means that the Defender has to give an Opportunity Attack to the Dragon, just so that he can move to the downed Rogue and give him a healing potion.
I suspect that some PC deaths could be prevented if all of the players at the table worked harder at assisting each other and less at playing the PC that takes down the BBEG. It really is a group game and everyone in the group has to be flexible with their actions and not just "I attack again this round".
Having a hard time hitting the monster? Then do an Aid Another for the Striker. He'll probably do more damage anyway.
Party getting pounded on by many foes? Have all 5 PCs attack the same foe to decrease the number of foes. Rinse and repeat.
Death can occur even with the best PC tactics due to bad dice rolls or a mismatch of the monsters with the PCs. But, the two areas the players can focus on the most to minimize PC deaths are their tactics, and their PC builds. That extra one point of damage by the Invoker can sometimes mean the difference between a hard fight and a killing one.