In a simple answer, it reduces the value of your life as a human to nothing.
Afterparties, trips to IHOP after bar hopping, post-concert burrito eating, these are all true 'afterlives' of larger events. They provide a nice ender, a counterpoint to the activity that came before (except for afterparties, you just keep going
). They are not greater than the event that came before - a short way to wind down, get a bite to eat, discuss what happened, etc.
"Christian Heaven" is not, I believe, an afterlife. It is your life, as it consumes the majority of your existence (being infinite - all of your existence, really). It reduces your life on earth to 'before-life' and nothing more. Anything you accomplish here as a human is inevitably pointless, since once you get to heaven, you will not need anything, for God is perfection. Heaven removes any need for humans to create, be it more children, art, new technology, love - God, in his perfection, is greater than all of that.
I had an older brother who only lived a week and a half. I had a cousin who didn't live to be born. Christians believe that unborn children automatically go to heaven - they have to, I mean, sending unborn babies to hell is pretty harsh, right? Even though the Bible actually says otherwise, let's just assume that they do all go to heaven, regardless of their parent's beliefs.
What happens next? They can't mature, since there is no environment in heaven which would promote growth. Even if they could 'grow up,' what age would it be to? It is our lives that define us, that give us our characteristics, our sense of humor, our wants and needs. These children will never have a way to define themselves, they will be 'perfect angels' or something similar. Another blank face in the crowd of billions.
I guess my question would be - what do you do in heaven? How do you define your 'real' life, the one that is to come?
For me, my life is here. It will end someday, and people will be sad, and that will mean it had meaning. Christians shouldn't be sad at funerals - it's like they're crying because the beginning credits of a movie are over.