I have made senior members' usernames regular font, and made the user titles bold instead. The bold usernames were quite chunky and space-consuming, the new style is sleeker.
It also helps close the perceived gap between senior and non-senior members, as I'm sure it can be a bit intimidating when you're the regular-font username guy posting in a thread where everybody else has a huge, chunky bold username, while at the same time keeping a visual distinction between senior and non-senior members, albeit not so drastic.
Think of it as a move to "lower the barriers" to participation.
