Wow. Another nerf LeBlanc thread. Unless she gets so unbelievably fed that she has all 6 items before anyone else does, she won't scale. Exaggerated, but she needs to be fed to do anything late game. There are a lot of things that can counter her. Talon is ad and can close a gap and silence in one move, better than LeBlanc can do. Not to mention he can go invisible and then kill her from there. Morgana kind of because her shield can block a spell or two making it hard for LeBlanc to land a combo. Soraka, I can't tell you how bad she counters LeBlanc. A silence, sustain, and a magic resist buff. Ap Soraka will own LeBlanc. Kassadin has a silence, is more mobile, and takes reduced magic damage.
Telling LeBlanc not to burst people down as an assassin is just plain stupid. Tell Vayne not to do tons of damage or tell Amumu not to stun everyone or Darius not to execute people.