Am I the only WoW player in the existence who doesn't want a Death Knight? Sure, they seem totally hawt an' all, but if you have to sacrifice your old character for it, instead of it turning into the Death Knight or lvling a new character from 1, I'm not intrested. I love my characters, and would never change them for some new, bright an' shiny character handed to me for doing some quest line. 
You don't have to sacrifice your character. Hero classes do not work that way. You can complete a quest PRE-60 (**THIS IS NEW INFORMATION! Previously, they said at level 80, now it's around 60 the quest will take place) to unlock the ability to create a NEW character, a NEW death knight. This character STARTS at 55+ (this is done because we've all already leveled up several times by now, most players do NOT want to have to experience old-game again. As more expansions come and go, getting from 1 to level cap is going to become more boring and tedious everytime we do it. So making you do a long quest is their way of allowing you to skip those levels, to start straight at the expansions). So you CAN have a death knight AND your current level 70 (or 80, whatever level you get him to).
Next off, Unholy Embrace in my opinion is not overpowered. It will likely be their level 80 (or 70) ability. Our hp's tripled in the expansion, I find it likely they're going to increase significantly at level 80 in epic gear yet again. Also consider everyone else is going to get new 51 point talents as well as new level 80 spells that are likely going to be equally powerful. I suspect healers will see some new abilities NOT healing related (instant combat res? Buffs that avoid death for X amount of time, allowing the character to continue fighting? Self heals that ignore debuffs (unholy embrace, stuns, silences)? Imagine what your class may be getting at level 80 and with 51 point talents. Don't fret, I'm sure Unholy Embrace will be very powerful, but it will not be the only powerful ability. Also consider paladin bubble, which is amazingly powerful - but it does not make paladins unstoppable. Nor does Ice Block make a mage unstoppable, or PoM Pyro a fire mage unstoppable).
Don't fret people, this expansion is going to change your class - but it may be good. We all are use to the new abilities we got with the old expansion. We'll get use to our new abilities that will likely change the way we play yet again.
Edit: And let's be honest, saved for a couple overpowered combinations in 2on2 (Shadow Priest/Warlock for example) healers are almost mandatory in the arena. Theyre a bit overpowered when two dps'ers blowing cooldowns can't down a healer because of line of sight issues and rediculous healing over dps output ratios. A spell directly built to counter healing on a long cooldown is, at this point, overdue.... not to mention some kind of balance to the line of sight issues going on.
