I am sad that the Rubinius team won't join, but I understand that they have seen it all crumble before, and don't want to invest in a time draining project again.
I'd also like to see some funny or surprising differences between the implementations, it could be fun to have a list of that somewhere.
Rubinius doesn't want to join is not because they don't want to invest the effort, they actually invested a lot of efforts before.
That being said, they bring up this project more for political reasons, that they want to control the language, e.g.
If MRI fails on the rubyspec, whose fault is that? It would be MRI if rubinius controls the project.
Now rubyspec is part of the ruby (MRI) project, and they have made a lot of changes to conform MRI, and in return rubinius would need to adjust to conform the specs, for a project that has 24K commit and still doesn't work on windows, you can imagine why they don't want to join effort.
I'd also like to see some funny or surprising differences between the implementations, it could be fun to have a list of that somewhere.