The PvP system alone made BC 1000x better than the original game. It was fun being able to strut around with the highest title, glad in this case(not on my shammys :P), without having to play 50 hours a week.

My favorite PvP changes:

Daily PvP quest.
AV - Reinforcements, Report AFK (20+ afkers at 3 AM kinda sucked), simplifying (remember how many random NPCs you used to have to kill?), and moving the horde base.
Resielience - My first TBC character was a priest. Without this little stat I wouldn't have been able to break 1800 let alone 2300.

*Edit*

Forgot to mention, not having to get 40 people together for a raid = win.