EDIT: Jim just beat me but I'd already written this so I'll post it anyway.
time-lord wrote: is there a compelling reason to upgrade to the newer versions?
Support for 3.0 ends in 2 weeks time.
MODs have been replaced with extensions which means to add a feature you just upload a folder and activate it. No more messing about with coding. This also means that when you need to update you can just replace the old files with new ones, run the DB update and you're done.
You're perfectly entitled to stay on 3.0 if you wish though.
Also I wanted to add language packs as well as add a new style or two to the forum.
Both just require upload and activation. However if you have a lot of MODs you will need to code in the style changes to the same files in the added styles.