This new expansion doesn't revolutionize WoW, but just adds bits and pieces that improve on an already great game.
The new land of Northrend is a worthy addition to the game. Just reading through it description in the manual, you would think that it would be nothing more than a frozen mountain range similar to the Alliance area around Ironforge, but there is actually quite a bit of variety. Not just between zones, but even within each zone itself. Just in the first one, there are at least five geographically different areas. Many of the quests are more story-oriented, which I preferred. Players that just want to rush through and get to level 80 as fast as possible may get annoyed by this because there are several scripted segments that you must watch before you can continue some quests. These scenes are cool, but I would venture a guess that well under half of the game's population actually pays attention to the storyline. These scripted segments may also get repetitive when playing any additional characters through the new areas. More vehicles and mounted weapons have been added to some of the quests. One had me firing a cannon from the deck of a ship while another gave me full control of a makeshift tank. These features wouldn't hold up in an action game, but they do a lot to spice up the gameplay here. The graphics may not noticeably look better, but all it will take is a quick run through any of the starting areas to see how much things have improved. The style hasn't changed, but everything looks more vibrant and detailed. There are a large number of rehashed enemy models from the original game, but not as many as the last expansion. The game still stands up well against it's competition, which is impressive since it is still using the same graphics engine it started with over four years ago. Basically, if you are still playing your level 70 character, you have every reason to pick up Wrath of the Lich King.
The other major addition is the new class, the death knight. Unlike the other classes, the death knight starts at level 55 and cannot be created unless the you already have at least one other character already at that level. The death knight is basically a warrior with stronger abilities. They are mostly a melee class but have some fairly devastating special attacks. What makes them different is their use of runes. Each has three types of runes and two of each type. Most special attacks use a rune of their corresponding type and then the rune must go through a short cooldown. It doesn't take long, so by the time you use the last of your six runes, the first is ready to go again. As you level, stronger attacks may use multiple runes at one time. The death knight has some of the strongest attacks, can wear plate armour (the best in the game), and can even heal themselves. All this makes me wonder if they might be a bit overpowered, since no other class is so well rounded. There won't be any way to know for sure until more people reach level 80 and I begin to see them in some PvP action. As for right now, I can tell you that my friend with whom I usually quest with was fighting just as effectively with his level 68 death knight in basic gear as he was with his level 70 hunter in full battleground and arena gear. The death knight starting area is, by far, better than any other starting area in the game. Every quest is very important to setting up the return of the Lich King and the entire segment has a very epic feel to it. It is slightly disappointing that it ends with you being dumped into Orgrimmar without any direction to go from there. All you can do at that point is head over to Outland and work your way up in levels. This is disappointing for two reasons. First, most everyone is tired of Outland by now and wants to jump right into the new area. Second, since the expansion came out, Outland is basically empty of other players. I flew around the main city, Shattrath, on a Saturday afternoon and I didn't see one other player. Both of these things could have been avoided if the death knight began at level 65 instead of 55.
There are still a few existing problems that should have been addressed. The first thing I ran into was the ridiculous respawn rate just outside of the starting area. The enemies are rather close together and once you kill two or three, the first is already reappearing. Once outside of this area, the respawn gets more manageable. Another major issue I had was that less than one hour into the new area, I was sent on a boar hunting quest. Either Blizzard is running out of ideas or it is some kind of inside joke with the developers, but there is no reasonable excuse as to why we have a quest to go kill boars in nearly every single zone of the game. There is also the return of the quests where you must collect an item that you would expect to be found on every enemy of a certain type, but it only drops off of one out of every five or so. These are really the only problems most people have with the game, but it is disappointing to see that they have persisted through two expansions.
One more thing to keep in mind is that this expansion has no content for anyone under level 60. The last expansion encouraged people to start new characters from level 1 while also having an added area for high levels. In Wrath of the Lich King, you can't get any of the quest in the new area until you are level 68 and the new character class is only available to high levels. If you are just starting out with the game, there is absolutely no harm in holding off on this expansion until your character has made it into the sixties.
Wrath of the Lich King is basically more Warcraft. If you didn't like it before, you still won't. If you can't stop playing, you'll still be hooked. If you quit playing just because you ran out of things to do, then this will give you plenty of reasons to start up again.