Rules is extremely powerful app, but not the most user friendly, that's a known fact. This prevents it from getting popularity it deserves both with the members and other developers. My suggestion here is to fix the issue with user friendliness, of course if it is possible. So, we can export our rules as XML and give them to others. shortening the path for them, but unfortunately this does not improve the user friendliness by much. You can serve a perfect rule on a plate, if the other side does not understand it, they won't be able to make the small tweaks required to customize a rule for their use case and we are back at the start.
My suggestion is to build another layer in the app, allowing us to export rules as a plugin with settings. So the other users won't have to modify the events/conditions/actions, they will just change a few settings that we expose to them.
For a simple example lets take the "send member a welcome pm" use case. Right now you need to create event (member is validated) and action (send the pm from this author). The last sentence is in foreign language for almost all people. It will be much better if I can create the "rules" logic and export it as a plugin that has two setting fields - one for sender, one for the message and thats it. The end user just needs to have the core of the Rules installed and they don't have to care about events, conditions and actions, this is all taken care of under the hood.
This will allow many of us to share our creative rules in a way that is simple for the others and won't require excessive support. Right now I do have some creative stuff (I think) and I do have even bigger ideas. But I definitely don't want to provide my ideas on the marketplace and be forced to deal with all the questions that will come with the more complicated rules.
So it will look like this:
Kevin - develops and supports the core, everyone will have to have this.
People like me - provide creative rules as plugins. The heavy lifting of the development will be taken care of in the core, we can use our creativity to develop final solutions for the ordinary users. We will also support our plugins with answering questions, improvements, etc.
Ordinary users - they will buy the plugins and the core. So no contribution from their part, except increasing the popularity of the app and the plugins and providing monetary compensation to the other two layers.
I hope this is doable and I am sure it can bring some life in to the application, because I frankly think right now it is almost abandoned. There haven't been any new features related to 4.2 and 4.3 and even though @Kevin Carwile supports it with the rare bug fix here and there the development to me is far from where it needs to be.