There is a lot of forum software out there, but it seems to me that a lot of the popular choices are PHP-based. And for my project, I'd like something based on Java servlets so my team can make customizations to it.
Another important feature is that I can completely change the pages to hide unwanted elements without too much work. So I'm looking either for a template system or easily editable scripts (i.e. JSPs) that have a clean view separation. Just having skin changes or CSS customization is not enough. I understand that if I have open source, I can change anything I want, but my point is that it should be easy and not requiring mastery of a complex code base.
Finally, I want something that has been around for at least a year and deployed on some high-traffic sites. Clustering support (one database, multiple web servers) is highly desirable. Up-time is crucial since I have an SLA to support.
What do you think?