[Update 1: Added #4, Updated #2] [Update 2: Added #5, Updated all]
I don't see a business model here. I admire their desire to not sell ads and not to sell data, but then, they say "From time to time we offer special features to our users. If we create a special feature that you like, you can choose to pay a very small amount of money to add it to your Ello account forever." I am totally unconvinced that this scales. See: app.net
It looks like it uses the Twitter model of asymmetric links: if I follow with Jon, Jon does not have follow me as well. It is an interesting choice; not wrong, not right. Interestingly, this gives Ello more data about their users; the choice to make it difficult to find out who is following me makes the system less transparent. Hopefully I am wrong about the difficulty of finding your followers.
After a brief experiment, it turns out that it is not at all obvious to figure out who follows you. It may be a feature that's right there, but the Ello UX is so, so much fail. This seems like a major problem. You do at least get a notification in your feed and (if set up) by e-mail. (Update: It is available. Just buried. You have to go to your profile page.)
Second thought on the business model: "you can choose to pay a very small amount of money" Sounds suspiciously like in-app purchases. It's the Candy Crush Saga of social networks!
TC points out that everything is public, and there is no privacy settings. I'm not sure if this is a bug or a feature. I don't have a cite, but I'm sure there is some work showing that people really don't understand privacy settings very well. By making everything public, it eliminates a point of complexity. Further, look at all the issues Facebook has had in changing default privacy settings. Privacy settings are hard to do well. Changing defaults is impossible to do well.
Again, this makes Ello less a Facebook and more a Twitter.
Repeating myself. It's Twitter but with a Facebook-style interface. See thoughts here about how this will end. (Hint: Rhymes with "Beers")