Thought about this crap a bit more. Again.

Display
I’m pretty adamant about the persistent display, but there’s one big issue: dark environments. Light emitting displays (backlit LCDs, LEDs, etc) would either have to dim significantly or perhaps just shut off below a certain ambient light level to be acceptable in something like a movie theater.

Then on the other hand they still have to be bright enough to be easily visible in sunlight. Backlit LCDs are a bit weak at this, while reflective types (including transflective) are great, albeit not without issues image quality wise.

Another issue is display size, which affects other usability aspects. One thing I had in the back of my head was the possibility of a device meant to be able to be used on and off the wrist. I was thinking of the camera at the time (take it off and hold like a traditional camera), but there’s more possibilities beyond that. Apple has some funky slap wrist band patent, and flexible screens have been at trade shows for years. Combine the two and you can have a larger display without compromising too much on form factor. Then once you have a larger display…

Keyboard
…typing on it may become usable. Use text snippets on your arm as quick replies, or take it off and type as usual. It’d still might have to be a fairly substantial chunk of device on your arm though, so I have doubts. Then again some people like them big ass watches that are already out there.

Everything else (again)
And a bigger screen opens up the options and usability for whatever else out there. Increased visible display space, and increased physical touch space. It’d still have to be small by design, but when you’re going to be dealing with a tiny space to begin with, any decent increase in size is a huge deal.