The exact same game with a new coat of paint, but that's okay - you have to buy it to keep up.
Burning Crusade is an expansion that isn' t good because it doesn't have to be good. Either you enjoy WoW enough to continue playing it and will appreciate the new content or if you already bored with WoW, this expansion will bring absolutely nothing new to the table.
Burning Crusade introduces several new things: new zones, new quests, new raid instances, new races, and some new skills. The gameplay, however, is exactly the same. You will quest to level 70. You will grind 5 mans - each boss kill is a pull of the slot machine until you feel confident enough to raid or PvP. At this point, you can do the same three battlegrounds (each with one map), or one new battleground which plays like Arathi Basin or an Arena, which is a small-team sudden-death fight that also provides superior equipment to the battlegrounds. Raids are identical to the original WoW expansion, only with 25 people instead of 40, which makes it easier to organize (assuming your guild survived the "downsizing paranoia"). I will also note here that the new skills will not change the playstyle of your class in the least.
The the bulk of new content exists as new graphics, which I can only describe as completely unoriginal and uninspired. Upon exploring the many new areas Burning Crusade had to offer, I could only imagine the following conversation between the art director and an artist.
Artist: "Take a look at this armor concept, we thought it looked pretty good."
Art Director: "MORE CRYSTALS."
A: "But sir, there are already crystals on the shoulders, and elbows, and circlet."
AD: "MORE CRYSTALS."
A: "I don't think we can fit any mo-"
AD: "MAKE THEM FLOAT."
...you get the idea. At any rate, with perhaps the glaring exception of Zangarmarsh, you've seen this "chaos world with floating crystal and rocks" garbage in every recent Final Fantasy game made - and it wasn't impressive there, either.
In short, Burning Crusade is a textbook expansion for an mmorpg. It is mandatory for progression, adds no new gameplay elements, but does provide new areas and items to chew on. If you just can't stop playing WoW, this won't ruin anything, but don't be fooled by the drooling WoW-addict reviewers; there is nothing to bring you back either.
~Petrovsk
(Note: This is an assessment on the expansion, not the game as a whole. WoW is still a fine MMORPG if you have never tried it.)