The consistent interface point is well taken, and I think that's the main reason for OO's commercial success. If you force everyone to be verbose in their interface definitions, code is far less likely to break across distributed teams of devs.

I take issue with your "functions/methods are data." remark. They are not. If they were, they'd be mutable, and I could process them, make copies of them, and change them the way I do other data. This is the wannabe lisper in me getting out, but there's a world of difference between treating your methods as data that you can pass around, and having them be data items that you can actually transform.

In C I can pass function pointers around, and do with some regularity, without making everything in my programs objects.

I realize that some OO languages have this ability, but the mainstream ones (java, C++, python, ruby) generally do not.