Identity is solved by real-world trust. Facebook has proven most of us are separated by less than 5 degrees.
Also, I have only about 15 apps, 300 pictures, and 10 sorry videos and I'm constantly running out of space even with half the stearate being on an SD card. I can't imagine puerile having enough storage to run something like this on their phone.
Most knowledge as we use it now is ephemeral and can be somewhat centralized safely. The permanent knowledge or at least the trusted hashes of that knowledge can travel with us.
- Data plans. Everyone connecting to your blog through your phone would use up a hell of a lot of data.
- iPhones are notoriously hostile to P2P apps, so that rules out about half of all smartphone users.
There are other issues but those are the most significant imo.