I've seen it all too often, both in professional and novice MMO's. You're fighting in a high-level dungeon, barely holding off mobs that are 4-6 levels higher than you, and for that large difference, you're doing pretty good. You're dealing out quite a bit of damage while staying out of trouble just enough so that you don't annoy the healer. Suddenly, you find yourself dead, and you aren't sure what happened. Did a member of the opposite faction sneak up behind you and snipe you with an arrow? Or did a mob spawn off to the side of you?
No, you're tank and healer experience a bout of lag, therefore transferring all of the aggro over to you. And your 500-HP butt couldn't withstand a blow without keeling over.
The healer endlessly apologizes, complaining about the modem and the ISP (Internet Service Provider) and everything that could have possibly interfered with his connection (Meanwhile, the tank is still warping around the zone because of his lag). It's fine, though, because you lost a measly .5% (Yes, that is a decimal point in front) of your total experience, which is nothing compared to what you were getting per kill. You'll probably earn 10 times the amount you lost just by killing a single monster! Plus, you didn't lose any of your treasured gear, and you respawned at the beginning of the zone. A simple minute-long walk will bring you back to where you were fighting, and then all will be well.
My point? In most MMO's, the death of your character does not carry a high enough price (Unless your cause of death is from another character through PVP. In that case, all that is affected is your kill/death ratio and PVP ranking, whatever that may be).
So, what would an ideal death system be? Well, one of the best death systems I have seen so far in my history of gaming is actually from RuneScape. While I may sound like a complete idiot for even mentioning anyting good about that game to some people, I ask them to think about it. In RuneScape, death holds a very high price. You don't lose experience, you don't lose any levels whatsoever, but you do lose all of your beloved items that you carried with you except for the three most valuable items. So, while that highly expensive weapon that you just bought may be safe, almost all of your precious, valuable, and good-looking armor is lost unless you are able to find your body and recover them.
However, the games we shouldn't idolize for their death methods could be Everquest I and II, World of Warcraft, Adventure Quest, Dragonfable, etc. I know for a fact that (with the exception of World of Warcraft, as I have only played the trial, so the death system may or may not change in the later levels) each of these games do not take away anything important except for a little bit of experience or perhaps nothing at all. The only real penalty is having to walk all the way back to where you died at and revive there to continue fighting.
The reason I am ranting on and on about this is that I am tired of seeing people who regard death as a "minor annoyance" in a game. Death shouldn't be realistic (for example, when you die, it counts as a permadeath, and you have to start over from scratch) but it should be highly undesirable for anybody of any level. They should lose something important to them, like an item, or perhaps even a hefty amount of experience, for those higher-level players who farm enough money to replace anything.
I highly advise the developers of MMO's to rethink the "traditional" death system and come up with a more substantial price to pay for those who love to die. After all, death isn't death if you don't "die".