When Plutohome were active, there was a commercial incentive and it was easy to install and use. I didn't even know what Linux was installed underneath, you just burned the two CDs, rebooted and it just worked. Dianemo seems to have the same philosophy.
I also have been following this project since PlutoHome times (back in 2005), but actually I can't remember that it was easy to install ...
Also at that time it was very hardware sensitive, and more often than not the automatic update mechanism was breacking the system.
I can't count the times I had to rebuild from scratch my server because update had scr**ed the sistem up ...
I can't also count the times I had to recompile kernel, drivers in order to fit Pluto in my system.
So I would't blame that much on the actual system, that is beeing worked on also to squash these kind of difficulties.
I agree with you that probably it won't ever be something to hand to an average Joe, but I think that the same apply to any system that deals with some internal complexity.
I don't think that you expect that deploying a KNX system should be an average Joe task ...