Great thread guys - thanks for the input.
I am leaning towards a 'credit' based system where everyone can vote on the features they want to see. I'm considering incentives and benefits, such as giving users extra 'credits' for every year they've been with us, with a maximum to balance the field.
Then there's the issue of those who need features fast and are willing to pay. This is quite rare, and I don't see it interfering too much with general development. Were I to take on these jobs I would balance them with the top-requested features.
The new site, when it's released, will be the beginning of yearly subscriptions for updates and forum access (for new users) so I'm not as concerned about the revenue from features. I am also implementing "site licenses" although, this will not (yet) be enforced beyond the 'honor system'. Furthermore, resellers will be able to purchase licenses at a steep discount and actually create (and reverse, for 90 days) their own ListMailPRO accounts for their clients. I've also added some new services including an 'offsite backup' service where you might choose to pay $5/mo to automatically store a daily backup on my servers.
With all of the above I think we will be just fine for profits and I will not need to 'tax' feature development.
What do you think?