Freedom from the afterlife
It's a bit hard for me to relate to the sense of loss that Brad's budding atheism brings him on this point, because my own history is quite different -- to the point that I run the risk of having "wishful thinking" reasons for being an atheist. As a Christian, I experienced the idea of the afterlife as oppressive. I never had confidence that I was headed for heaven. I was terrified that I had committed some sin, failed to work hard enough and be perfect enough. I was terrified that however much I thought I had faith, there was some unconscious doubt that God would point out when it was too late for me to fix it. I was terrified that I had somehow, without meaning to, given Satan some unbreakable claim on me. And for all that, I would suffer for all eternity. The possibility of heaven only made it worse, because I could be blamed (by myself, by others, by God) for not doing what would have been necessary to change my own eternal fate.
My first step away from orthodox Christianity was to focus on several passages in the New Testament (I don't have time at the moment to try to find them) in which Jesus seemed to be essentially saying: "Don't worry about the afterlife. You can't know what's going to happen. I'll take care of it for you." This was not a traditional Christian "good news" of having confidence Jesus would make sure you go to heaven. This was a liberation of sorts from having to think about the afterlife at all. I could go about being a good person, expressing kindness and love in the here-and-now, and whatever happened after I died was not my concern.
But the more metaphysical my conception of God became, the less it really fit the name "God," until I finally had to admit to being an atheist. And now the thought of death is not a burden on me. In fact, if someone could credibly offer me eternal life, I'm not even sure I would take it.
The afterlife -- at least in Christianity, which is the only tradition I know in detail -- makes one's past a constant burden. What you've done, for good or ill, constantly accumulates as baggage directing your future. This feels oppressive to me, no matter how sure I am that I've accumulated good baggage.
I don't want to die anytime soon. Were I to be hit by a bus on my way home tonight, it would be a tragedy from my point of view. I'm attracted to the idea of living a full life, then bringing it to a conclusion and choosing to depart once you've had a good run. That conception strikes me as more beautiful than any promise of eternal life. I'll take my stab at life, then sign off before I wear out my welcome. The world can choose to preserve, build on, or forget my legacy as they see fit, but I won't have to be burdened by it forever. Let life be carried on by new people, who come in with a clean slate. The best thing I can do is to make sure that those who come after me have the best opportunities for what to do with their life, rather than accumulating credit for my own accomplishments.