1. Self-contained, untethered synchronization, much the same way a Blackberry gets email.
2. Read-write, two-way, should be able to record and connect with a publishing system for automatic upload and feed production.
3. Must be a platform, that is, people other than the manufacturer can add apps.
About the third one, to be a platform, I wonder if being able to read RSS and OPML and let users access that would be sufficient? In other words, the device can access OPML directories and RSS feeds, and that way anyone can make directories and content available.
Or do you need to be able to program it? I’m not sure how much extra hardware/complexity/price that would involve (would it have to be a webserver itself?).