I'd say the two main questions I have in mind when I blog are, "what can I as a layman do to make my beliefs more reliably true?" and, "what things are important to think about".
Recently I though of a way of conceptualising trying to deal with both goals in choosing what to write and think about. Imagine a function defined by the difference between what I believe and what people in my social circle actually believe. I blog with the intention of maximising the first derivative of this function (setting the second derivative to zero). This could involve me changing my mind or people I know changing theirs, either way.
So take beliefs about global warming, I agree with basically everyone that it's happening and humans are responsible, so f(t) = 0 and this has been stable for some time so f'(t) = 0 too. So I don't blog about it. When I watched An Inconvenient Truth, see yet another global warming book on the shelves or watch a nature documentary I get frustrated, not because I disagree with the factual stuff, but because everybody already knows and agrees!! I'm guessing that this kind of book/documentary would often be defended on the grounds that many people (like republicans) are global warming deniers. This is true, but if changing their minds is the goal, why are these products so nasty about these people? So I think I come off as a climate sceptic, but I disagree mostly with the approach many environmentalists take to the debate (I think the mainstream should more about carbon taxes than drowning polar bears). So since my beliefs about that topic are different and that's what I'm more interested in talking about.
My beliefs about god are very different from most people I know so f(t) = (large amount), but this difference is also pretty stable so f'(t) = 0. So I don't blog about it much anymore. I used to blog about it more, but that's because I hoped to convince people to change their beliefs. I may still want that, but accepting that it's not going to happen lead to less blogging and thinking about that.
Immigration, economics, politics and vegetarianism are all topics where my beliefs are very different from most of the people I know and I believe that there is more scope for some convergence in views (my views on economics and politics are more fluid than my views on immigration and vegetarianism, so I don't always blog with the sole intention of changing other views to mine).