Totally agree with GGG and Squiglet, there will be a 'cull' of the population at some point. There is no way we can continue populating the Earth as we have been.
If (and I think its a big if) the human race survives things need to be very different.
These are really uncertain times as to our future. The west is on the brink of financial ruin and its own decadent downfall whereas China and the like are raising up out of the depths of poverty and dictatorship. Religious differences between us are scarily vast and its seems more and more certain that all theses beliefs cannot not live side by side in harmony. Something at some point will give. I fear however it will be mans own doing.
So for starters I don't think religion will exist, for various reasons (which I won't go into).
Technology and science will be extremely advance. Like Squig says, you only have to look at how much its developed in the last hundred years to have some idea of where it's headed. Unfortunately technology advances more in times of war. Hopefully we would have left our planet, here's hoping it isn't only to mine others
But more excitedly we may have contact from similar developed species.
I'm beginning to believe more and more that marriage will die out. Statistically speaking humans have a real problem with monogamy!
No one will smoke, I think they will be horrified by the idea of smoking.