There's a lot I could say about Ello that I know has already been said, so I'll refrain (for now at least), but one thing it looks like this place could be particularly good at is longer form updates.
If Ello introduced a few typographic presets to make the reading experience a bit nicer I think they could encourage people to post some really creative stuff. Maybe just start with a nice serif post style and a modern sans-serif look so if people want to write short fiction or blog post style updates it'd be a better fit. Ello could occupy a space somewhere between Medium, Tumblr and Twitter (technically if not culturally). This mono typeface would function as the status update default, I suppose. The 'aside' format.
Twitter has many people who are very creative or informative within the confines of that medium, but it feels like somewhere with a bit more breathing room could be a good thing. Perhaps the only thing I really took away from App.net was that the 256 character limit was much nicer. It felt right.
Anyway, I don't have a nice conclusion in mind, so this is where I stop.