The default icon does have an impact, but not its quality so much. Rather its content: if it is witty or beautiful or whatever that can tell me something about the person's character. It serves the same function as email signatures do where I started online (much of my fannish communication is through email-based listservs).

More than that first impressions, though, icons definitely flavor how I react to a post. If something is longish or if it's behind a cut, I'm much more likely to read a post with an interest-catching icon. Go figure.