Since this post, I've been contemplating why having a crowd in shul matters. Haven't I been known to argue that the Temple altar has been replaced by the home table; that Judaism is something we can, and should, practice in our homes? Short answer: sure. But home-based Judaism is complemented by congregational Judaism. And although a home ritual can be perfectly valid done solo or in a family context, a synagogue service needs a crowd in order to feel right. In fact, it needs a minyan.
The Torah reading can't happen without a minyan, for instance; we can discuss the parasha, but we can't actually open the scroll and read it without a quorum of ten. I'd like to think that there's a reason the Rabbis declared it that way: that they knew some kind of communal energy accrues when there's a critical mass present. That we davven better together (though one can pray just fine alone).
One can pray just fine alone; but I've noticed that I don't, much, unless I go to shul to do it with other people. Sure, I chat with God once a day, but that's not the same experience as settling into the arc of the liturgy, start to finish. It's like yoga, which I technically know how to practice, but which I never do alone: I need to join a class or my mat languishes unrolled. Alone, I can always find excuses not to engage; but if I'm meeting other people at a designated time and place, the obligation makes me show up, and once I'm there I'm glad.
Prayer is also qualitatively different in a crowd. When I'm depressed, for instance, singing familiar prayers with other people soothes me in a way that singing them alone doesn't. In Friday's erev Shabbat service, I found temporary respite from my anger about the abuses in Iraq. Praying for peace with my compatriots won't actually change the situation, but it will change me, at least for a while. That's a spiritual transformation I'm not capable of accessing alone.
Praying with others is an antidote to isolation, and that might be another reason the Rabbis legislated the need for a minyan: to keep us from splintering off and practicing our different Judaisms alone. We need to keep speaking to each other, because we need each other to pray with. Like a parent whose will mandates that the children share the estate, thereby ensuring that familial connection remains, the Rabbis mandated that the children of Israel study the Torah together and pray the liturgy together, ensuring the continual re-creation of our communal bonds.