(First day use braindump)
So far, most of what I like about Ello is the on paper stuff. Even then, though, that list is somewhat limited. I appreciate Ello's focus on privacy when it comes to the user/advertiser relationship, and this is a great starting point.
However, I'm not seeing a whole lot about inter-user privacy on here. Right now, I would not recommend Ello as a safe space to, well, anyone. It's frustrating that I have yet to see a social network with the privacy/access/sharing controls that livejournal has had for a fairly long time, now. It's been over 10 years, folks!
It feels like Ello has gotten a slingshot of activity as a result of facebook's war on names they believe to be not real. However, so far, the only thing Ello has to offer on that front is "we aren't requiring legal identities".
I'd love to see a social network that started from the premise that a user's information is their own to control. Start from the base of allowing a user absolute control over who can see what and when. Allow someone to have an account that consists entirely of hidden data, of posts that only they can see, hell, don't even acknowledge the existence of said account if the user wishes their account to not be visible at all. Start by allowing people to form very tight knit, private communities. Give people safe spaces to gather and build up social networks that they control.
Most social networks up until now feel like they've run with the mantra "what is not explicitly made private, make public." I'm waiting for one running with the mantra "what is not explicitly made public, keep private."
Anyway, I'm often interested in new sandboxes and I'm usually curious to see what things a new social network will bring to the conversation. I'm cautiously optimistic about Ello, but I think that's mostly because I haven't stumbled into anywhere else just yet that's caught my attention. Right now, except for the "we promise not to collect/sell your data to advertisers," there isn't a whole lot to differentiate Ello from early twitter.
 Note: it doesn't count as "private" if every existing member of the site can see it. Just because people not in a private(ish) beta can see things doesn't mean things are private. The site's going to open to everyone some day, no?
(and as I attempt to post this, Ello goes "Offline for maintenance")