There are tons of ways you can end up dead, in an otherwise ongoing fight. You pulled aggro, didn't get out of fire, got randomly selected to get owned by some special ability - whatever. But your contribution to the fight does not end there.
Even while hugging the ground, you have valuable perspective on the fight - possibly even more so because you aren't worrying about DPSing, Healing, Tanking, ect. This is your chance to make a difference!
If there is some ability/timer/percentage being called out, that someone living is doing - take charge of it. The person relieved of that duty will perform better.
This is probably the most important thing you can do when dead. With the raid down a person, everyone else is going to need to focus even harder. Although a person outside the instance (but in the raid) can read raid warnings as well - they cannot see boss cast bars, boss health, or any visual only environmental hazards.
If there are battle rezzes / ankhs available - be prepared to accept/use them. Remember that it is not always smart to hit "Accept" as soon as it pops up - the person that rezzed you may have had to move out of some AoE since casting the rez, or there may be some upcoming raid-wide damage that a fresh, low health, unbuffed version of yourself cannot handle. You need to be watching for these things, lest you waste raid resources.
However - do not request a BRez ("Rez Me!"), or decide to Ankh, on your own. Calling for Brez'es or Ankhs is best done by the raid leader / an officer - though you may want to ask them "Do you want Yakra to be rezzed?". Don't be shocked if the answer is a gruff "No", if your method of dying doesn't warrant a second chance.
I can say, as a raid leader, that non-officers who take it into their own hands to make decisions about how raid resources (rezzes, wipe protection, heroism) are allocated - tend to do so poorly. There is a time when the raid leader knows its going to end up a wipe, but wants to keep going for practice - these are not the times for long cooldowns, though a normal raider may not know that. Trust in the decisions of your raid leaders is key, for them to orchestrate success.
A particularly irksome action, is for a shaman to ankh right after they die, without saying anything, to try and cover up a mistake. I once Gkicked a Enh. Shaman for doing this repeatedly on Archimonde - especially since all he did was soul charge the raid twice (he was terrible at not being on fire).
I've also had people walk away from their computer, or take off headphones, after dying - either our of frustration, or for some other reason. There was one case where a rogue proximity aggro'ed Kaz'rogal after trying to distract him, died, and then left his computer. He got Battle Rezzed, but never took it. While that didn't end in a kick (it was an isolated problem) - he got hell for it.
So - if you absolutely must take a dirt nap mid-encounter, do everything you can to lessen the burden of 24 manning a boss! You will look better for it (though not as good as if you had stayed alive :P )