It is truly "best in series"...
This time we dont get any new race,but we got awesom new class,a Death Knight,which represents me a paladin-warlock mix,as you have 3 "stances" and a lot of debufs and melee fights,so if you into something new,this is for you.Starting quest line is even greater than i imagined,you relay "feel" how badass Arthas is,just last quest ,when you become good could be more interesting :) Anyway,this is epic add for epic expancion!
Northrend,so far its much better than TBC starting place was,again with a lot of Q,which arent best ones,but i find them kinda interesting :) Still,many mmorpg's out there have "kill x creatures" quests,specialy chinese games.
Dungeons.So far The nexus is most intesresting for me,with elemental bosses,it was such a fun to enter dungeon for a first time and without any idea what to do clean it!.
Well,thats just few things which are on my mind at the moments,its worth to buy expancions,and dont listen to others who says its time wasting and etc,its not yours foult they failed at life and they think its wow's foult :) try it,just dont over-do it