The locker project had a very clear data model to share data such as contacts, statuses, links, places etc. But more importantly it was easy to sync data from existing cloud services. Looking briefly at cozy-data-system, it does not seem like it is designed for applications to easily share data.
In any case, this project and others like it are really interesting to me as a hobby developer. Having structured data that I create and consume on local systems makes it a lot more enticing to run continuous analytics on.
I've been building out some tools that pull data out of pinboard.in, twitter, facebook, last.fm, my past Google searches and then do some basic processing[3] to create a sense of what I'm doing, how ideas introduced to me, things I'm interested in over time etc. The locker project was a great help, because they can merge data from multiple services and then I can keep the data on my own servers. I would love to have all the data/applications myself, but do not want suffer with sub-par apps and use the best services available (as long has they have an API).
[1]: https://github.com/LockerProject/Locker
[2]: https://github.com/Singly/hallway
[3]: For now, it's a mess of python with some nltk.org, www.clips.ua.ac.be/pattern and recently pandas.pydata.org