Actually, they have already confirmed individual attitudes and personalities as being in the game. They described the very different experience you will have dealing with France under Napolean-as opposed to Louis XIV. They also mentioned how Ghandi will tend to aid weaker nations, and will not go to war at a whim. The other differentiation is that each leader has his/her own set of preferred civics options and building priorities-which will come across not only in AI behaviour, but even in how you the player will play that civ.
The last part, which is speculative on my part, is that each leader will also have their own Unique Unit.
So, if I understand correctly, what will set leaders apart is:
1) Their Traits.
2) Their Civics and Building priorities (and, possibly, even their tech priorities).
3) Their attitudes and behaviour towards other civs (and possibly towards specific civs as well).
4) Possibly their Unique Units.
Yours,
Aussie_Lurker.