UU is a lot about secular humanism and respecting all belief systems (not necessarily approving of all of it, but tolerance is encouraged), and discussing them and discarding the stupid bits and gaining from the bits that resonate with you or that you find spiritually nourishing.
I have an atheist friend who will occasionally attend UU services with her mom or aunt because she enjoys the atmosphere of peace, contemplation and the focus on secular humanism.
I think that the idea of groups appeals to extroverts or social introverts who seem to have a larger need for this sort of thing. A belief system is just something to form a group around and focus activities around.
Extroverts, in the psychological sense, (and I am definitely an extrovert) need socializing and grouping up and interpersonal interaction to recharge their batteries, so I kind of like the idea of going to an unstuffy, liberal and ecumenical church like UU. As strange as it may sound to you introverts out there, socializing energizes us and being alone sucks us dry, depresses us and makes us very unhappy.
Introverts recharge by being alone, so they're probably the people who sit in a quiet corner in Barnes and Noble and find spiritual succor in quiet cup of coffee and a book, or sitting in yoga class contemplating their navel, or just maybe sitting alone in a cell praying, but extroverts would need a lively, open group discussion, a charity drive or a social occasion like a church dinner or dance to get fired up about spirituality.