The fact that it is - for all intents & purposes - an utterly complete, easily searchable database. Almost anything Lego I can think up I can find. If I can't think of a set number I can think of the name. If I can't think of a name, I can think of a minifig, or a part in the set, or the year it came out, or the theme...etcetera. The cross-referencing is invaluable! I used to use Bricklink for all my browsing, but it doesn't have as much information on one page as Brickset does.
Bricklink is a terrific site on its own merits, but if it were a programming language with all knowledge Lego, Brickset would be the GUI that makes all that knowledge come to life in spectacular fashion.