I have had to write a lot of emails to people lately, some to my professors. Although a lot of them tell us (their students) to call them by their first name, and I use that to address them in these emails, I'm still for the most part uncomfortable about it. It just seems informal somehow.
I thought at first that it's because I don't know them that well, but then again I don't know my TAs that well either, and yet I call them by their first name. Age might be part of the problem, since I do lack friends from that age group.
The larger part, I think, is the prestige. There are in fact a few professors with whom I'm comfortable using their first name, but I think they are generally more approachable. They tend not to stand at the front of the class and lecture, but would come by and talk about things in a more personal, friendly manner. This is not the same as a discussion though, but something in the manner of speech. What I mean by prestige is, I feel that some professors really do know a lot, and that by calling them by their first name would be disrespectful to that prestige.
Strange idea huh.