Beyond first impressions, avatars are also a very easy way to identify posters. I much more easily remember an avatar (and when I see it again I think hey, it's that guy) than I remember names. Even when I remember both, it takes less time to identify a user by seeing their avatar than by reading their name. This is the primary reason I don't change my avatar often. Sometimes when people change their avatars, I go 'wait, who was that guy again?'.
Frostburn wrote:What impression is given to an individual that has neither avatar nor signature and has been around for a while? I can't really place this in a first impression air since, by default, a new user will not have an avatar nor a signature picture and thus judged sorely by post content and user name.
Related to the above point, avatar-less posters will have to either post often or post very memorable things, or I'll just not remember them at all, treating their posts as if they were anonymous contributions. There's a lot of people on this (and on any other) forum, so if you're not easy to identify chances are I won't notice it's the same person when you make another post.