From WoWInsider’s How to PvP article:

Death is not a failure”

Honestly, if you only read one line of this article, make it that one. And this one, I guess, or you wouldn’t know which one to read. If you’re a PvE-er it’s particularly relevant. You’ll have the mentality of wiping, of death being the end of a pull, the end of a run, a failure. And this mentality will cause you to hate PvP. In random battlegrounds, where the majority of PvP takes place, death should be treated as a time-out.

You got a little ahead of your health bar, so you’ve been benched for 30 seconds to recover before you return to the fray. And yes, in an arena, death means that you may well lose. But learning from it is the key. What happened, why did you die, what could you have done better, what could you try next time? Try to see death as a learning experience. A flaw in a strategy, not a wipe, a complete failure. There’s a reason why there’s a “deaths” count on the scoreboard. Everyone else is dying too.”

The rest of the article makes for a good read as well.  

If you’re low on mana you’re near worthless.  Dying lets you come back into the fight with a full bar of mana.  That’s also why to call out the time left to resurrect if you’re in the graveyard.  That lets your team know they could die and come back quickly or would need to wait a long time.

If you have an enemy healer that is low on mana then don’t kill them unless they’re preventing you from capturing something.  They’re weak, let them stay that way.