I kind of have the same problem you do, adding a comment into a discussion feels natural to me, I can give a couple of points in the part of the discussion I feel most comfortable in and leave it at that.

With a blog I always feel like I want to do a well researched comprehensive piece spanning an entire issue. This is where I decide I don't have time to become the master of the area I'm thinking about and don't write it. It's that perfectionism that prevents me from really getting started.