"you know that there are many things which we will never know, including the existence of God (whatever that is), the existence of "things in themselves" (or Ultimate Reality, or whatever), and the existence of "true" knowledge. Thus, everything else on your list is an unknown and you are left with aesthetics."
Indeed, the aesthetic appeal of an idea is one of the only things you can know more or less "for sure" about it, however in order to function in the world we need to quantify the certainty with which we hold other issues as well. The question is "what criteria to we use to do that?" In my opinion, the only criterion, that is, the best we have, is to look at a theory or a belief on a purely objective ground (compare its predictions with what observation tells us.) So, though we cannot know anything for sure, we can be *relatively* more sure that the Earth exists than tachyons exist. Thus, there is more to the list than just aesthetics, as long as you are willing to compromise a bit.
"However appealing it might be, Christianity (along with other religions) has indirectly been the result of much suffering. One should surely eliminate it just on this. But there is something else to take into account -- it might have improved a lot of people's lives as well."
I don't agree that either religion or secularism is responsible for mankind's woes. Rather, our own inherent selfishness and corruption is, and we just use ideologies as an excuse to perpetuate this suffering. I'm pretty sure that under any ideology we'd be more or less as screwed up as we are now.
"All religions and spiritual ideas are equivalent in the agnostic view, they are all possible and they are all unprovable."
Yes, but be careful not to equate "all unprovable" with "equally valid." The better an idea fits current observational data, generally the better it is.
"People just can't get over the fact that they're not the center of the universe."
No kidding. Religion is man's hubris incarnate. If mankind is truly psychologically incapable of handling a world devoid of blind faith, I'll think about officially disowning this species.