A quick delve later and I believe now that it's to do with the 'sassiness' (their word, not mine) rating of my 'Fountain' (grind teeth at name). At first I thought that might be to do with how many friends I had or something but that appears not to be the case. It appears to be more to do with a combination of who invited you and how 'sassy' your blog (a mandatory field in the signup process) is - presumably obtained through Alexa or Digg or Technorati or something.
Since I, err, obtained my second invite by guessing the user name of a A-list blogger my second account was deemed 'sassier' (that's actually giving me a headache now) than my first and I get correspondingly more features.
How ... weird.