Wow, excellent site, very well done !
It is like we are having a competition in quality of presentation, marketing and documentation, cool. Things like this are really pushing the status quo forward.
What saving 'Models' means? why not any object? By Models Mapless means that you are not pretending to save transient stuff like contexts or sockets or filehandlers, etc. Any instance of a MongoMapless subclass will save in a breeze. Those instances are going to be serialized and stored as documents of its correspondant MongoDB collection.
Is only for tree-like structures or does actually support an object graph? Given that you follow its rules,like saving children first and thinking your models as noSQL friendly documents, yes, it does support an arbitrary object graph.
Why would I want to use Mapless? Because you might want to profit from some of these benefits: