It's been running for a while and as an open source initiative, there have been a lot of extensions and customisations proposed.
There's probably a lot of cool customisation possible as a result. I have already installed a couple of things, like a reputation system, a mod to customise the logo and the banner, as well a mod to put the user profiles on the left rather than the right.
The two main ways to customise the forum are:
- Extensions: new features. See here for an official list.
- Styles: new appearance. See here for an official list.
Aside from that, there should be a lot of cool stuff available (e.g., maybe also smiley packs for phpBB).
Aside from the pre-made extensions and styles, the code is open-source (PHP / MySQL) so we can also make modifications or even new modules (though this will be a lot more work).