Teetime that is exactly me, couldn't have put it better! I frequently prepare a longish comment, or start a new thread, preview the post and then change my mind because I'm unsure of the reaction I'll get. So I too just keep to the lighter stuff mostly.
I'm more than happy to get involved in 'heavy' conversations in person, but this is altogether different. Once you've posted your thoughts, they're there for all to see forever (unless you ask for them to be removed of course). And unlike a real conversation, anyone joining the thread later can read and comment on what you said in perpetuity
. If people joined you and a group of friends in a coffee bar, they wouldn't be able to hear everything that had gone before. It is a bit like baring one's soul
.