There has been a great deal of change and evolution of the world of World of Warcraft, and to a certain extent, all the available player races have gone through changes because of the events that have taken place. The original release content had lots of dungeons and quests and things going on, but each one seemed to tell the story of a place rather than the story of a people. Like each place, the stories told there seemed static, as the players grew and moved on, the places all remained the same.
The Burning Crusade, however, began to change all that. Instead of just adding new content with each patch, some aspects of the old content were changed as well, with certain characters and peoples coming to the foreground as major antagonists. Players were no longer merely vague adventurers tasked with saving the world from one giant evil monster or another, their characters had vested interests in bringing about some change in their circumstances.
For no group of player-aligned characters was this as true as it was with the blood elves. From the time The Burning Crusade was released, up to now, when the next chapter of the Warcraft story (Wrath of the Lich King) is starting to unfold, the blood elves are the only player faction whose leader has turned into a major boss in a dungeon (not once but twice!), whose capital city has been deprived of one of its most significant residents (who also ended up turned into a major dungeon boss), and whose culture has undergone a complete turnaround over the course of this expansion's expanding storyline.
The draenei, of course, played a huge role in the story of The Burning Crusade, but in the end, they were mostly just very strong supporting characters. The blood elves were the stars of the show.