Yes, its pretty straightforward to extend mojoPortal.
Yes, I've done most of the work driven by my own passion since 2004.
There have been a few community contributed features like the Poll and Survey.
I think one of the challenges is, before anything can become a core feature (meaning shipped with mojoPortal) it has to suport all the databases.
The YAF stuff is a funky integration that can be made to work for some dbs for some tenacious people, but I do not have the interest or bandwidth to support it, my included forums meet my needs.
Most essential features are already available and most people benefiting from mojoPrtal are focused on their own project or business needs not on becoming contributors or the spirit of sharing of open source. Its a rare person who has both the interest and commitment to contribute something more than a trivial feature. Most people only care about the free as in beer aspect of open source.
You suggest a need for a "Document Library", what is different about that than the Shared Files feature already included? What does "Document Library" mean to you? If you define it maybe we can improve the Shared Files module to meet your needs. A similar thing is people often request a "News Module" but when you ask them to define it it turns out to be a blog really or what they want to accomplish can be done with the combination of a blog and a feed aggregator.