You can read the full announcement in the news post at phorum.org.
When we officially started on 5.2 in March of 2006, we had several goals. I think we achieved most of those. We also accomplished some unplanned things. Our hackathon (me and maurice slept about 8 hours in 4 days I think) at MySQL Conference 2007 helped a lot. Remember, you can help us get there again by donating to our fund.
New API layers and more hooks
Better MySQL support
Improved bundled modules
New announcement system
New Search, inside and out
In addition to all this, there are things like more caching options, the new hybrid read view, the new feed options (the feed code was rewritten from scratch) and better error messages so that users are less confused.
We hope you all enjoy Phorum 5.2. It is the next step for this project that is over 10 years old now. And please, keep making Phorum all your own.