Death Knights have a few interesting passive abilities such as 100% crit damage all the time (as opposed to 50% for every other class, sure that can be boosted via talents), but the play style is very unique and, at least for me, great fun. They don't rely on mana or a similar type of energy. Instead their abilities cost runic power and runes, and are of course subjected to a global cooldown and individual cooldowns. Confused already?

So, the deal is you have 6 available runes: 2 blood runes, 2 frost and 2 unholy. Plus, runic power that starts from 0 and builds up depending on how much damage you do. Each ability can cost one or a few runes and/or runic power. Let's say an ability costs 1 frost and 1 blood rune. Each rune has an individual 15 seconds cooldown once used. So you can now only use 1 more blood and 1 more frost rune. You see why you must time your attacks properly and in the right order to be able to get in as many attacks as possible in as short time as possible. It's not as hard as it sounds but it's certainly something new and interesting.

Death Knights fill the role of tanks or dps, but you can do that on every talent tree; there isn't a dedicated tanking tree like protection for warriors for example. You just need the right spec.

They also make a very interesting combination with other classes in PvP, especially because of their high magic resistances, plate armor and the ability to bring targets in melee range with them.

Speaking of PvP

Many have complained that Burning Crusade was a healer's game in PvP, because with high resilience came an inability to kill healers with a powerful controlled burst most of the times. Players are now a bit on the other side of the barricade, where they claim some die too fast, and burst is too great. Of course, balance is a delicate thing, and I'm glad I don't have to personally do anything about it.

Strand of the Ancients is the new battleground added to the game, and it involves conquering a fortified position with the aid of the newly introduced combat vehicles, and then in a second round defending it.

Problem with that is there is nothing changed with the old battlegrounds, but still Blizzard compels you to play them in order to have competitive gear. Let's say for example I only like Arathi Basin. I can get as much honor and badges as I want, but in order to get a piece of equipment I have to do every other battleground and get the respective badges, because rewards cost a specific amount of each battleground badge in addition to honor.

So overall, the game has gotten more lightweight and many casual players are able to get PvP and PvE gear a lot faster, everyone walks with fancy epics these days, yet you have less and less freedom in my opinion: you have to do a little bit of everything, Blizzard kind of pushes down your throat everything these days.

Raid wise, instances can be played in 25 and 10 men, but you have to do them to stand a chance; and same goes for PvP, for every battleground and arena.