Wow how did I miss this for so long! Good thread.
My 2 coppers:
Flexibility is key. While I personally release all my plugins as free and open source, there is a significant portion of authors who would only contribute to the marketplace if their secret sauce is protected. Encourage but do not require.
Requiring glue code to be open source but having a well-defined way to wrapping up private code in a static lib would be the ideal compromise. Some plugins may also require external downloads (due to licensing) so any way of helping to accommodate that (via say a common downloader) would be advantageous. Also as mentioned earlier in this thread, having good tags will allow for customers to know exactly what they’re getting.
At the end of the day its all about minimizing customer burden in any way possible.