Changes were made to the Plus Usergroup promotion system. Registered users will now be promoted to the Plus Usergroup when they have over 500 posts and 15 reputation points. Those that don’t spam or otherwise cause trouble will get these 15 points pretty easily (note: reputation points are given by moderators).

Benefits of the Plus Usergroup include:

5 MB of storage space for attachments (instead of 2.5 MB for regular users)
ability to store up to 500 private messages (instead of 250 for regular users)
custom titles (text under the username)