What I particularly like is the functionality to process graphs and explore them interactively in the browser. This has been added in some recent version, and it makes working with graphs a lot easier than before.
Building a REST interface with access to your data is easy thanks to ArangoDB's Foxx framework. You can implement all your backend code in JavaScript and upload to the server.
Thus you can do any sort of preprocessing on the server and make that available to frontends. And it's easy to integrate with a front-end because it's all about passing JSON around via HTTP.
I think it's not to say that no relational databases has any of these features.
But I think it's more exceptional than the rule.
Relational databases are primarily designed for relations. And saving your objects as documents simply is a different model.
That doesn't mean one is better than the other, it's two alternative ways of achieving things.