There is no "list of features".
What you pay for is Premium Support. That gives you posting rights in the "Nabble Support" forum and Nabble undertakes to give you priority over "Free Support" users. It may not sound much but look through the Premium forum and you'll see how posters there get a prompt service and use it to do such things as request custom features for their forum.
These might be simple things like removing the "Powered by Nabble" line at the bottom of the forum page, to hiding the number of page views, to other things that are much more complex. If you're not an experienced programmer then you'll find having your own personal NAML programmer on tap is rather useful.
Sometimes what Premium Support users request get rolled out to all users as a standard feature, so some of the older requests you see in the Premium Support won't make sense to you because these days they'll be there already in the standard product.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.