I think standard "Junior Member - Member - Senior Member" is too short and also too boring. May be something with more steps?

For example:

Newbie: 10 or fewer posts
Commoner: 11 to 50 posts
Gentry: from 51 to 100 posts
Aristocracy: 101 to 250 posts
Nobility: 251 to 500 posts
Courtier: 501 to 1,000 posts
Serene Highness: 1,001 to 1,500 posts
Royal Highness: 1,501 to 2,000 posts
Heir Presumptive: 2,001 to 3,000 posts
Heir Apparent: 3,001 to 6,000 posts
Majesty: 6,001 to 10,000 posts
Imperial Majesty: 10,000 posts and above