Destruction ofcourse, it is great, unlike rs6 one which encourages camping, bc2 destruction was made to counter campers that hide in buildings. Weapons ttk and other stuff were good too. It also had server browser inside a game, which seems difficult for modern dice.

Game has a lot of problems too:
net code
weapon balance
map balance
team balance
campers
terrible car physics

But still bc2 remains best battlefield for me.