Very little configuration required seems to be one main idea. The most important reason I have been interested in Tox is that I got the impression early on that they intend to make it resistant to traffic analysis as well (and judging from this thread http://4chanarchive.net/threads/g/Tox-Thread/43340618 there are still such plans) but that isn't emphasized in the page at tox.im any more.

I'm guessing they're prioritizing getting something that the average Skype user would be capable of installing and configuring, and saving the onion routing feature for later.

I do think that a directory service for search/lookup may be desired as well... where people can upload their device key(s) and bind to say their email address or other handle for search/discovery.

Tox is peer-to-peer, whereas XMPP requires (de)centralized servers. Tox is encrypted always, XMPP is unencrypted by default and OTR is hard to use. The Tox API, as alpha as it is, is pretty easy to work with, too. I wrote mostly complete Guile bindings for libtoxcore and a basic Tox bot in about a week.

You don't need to run a server.

You don't really need to run an XMPP server either. There are plenty public ones available, and the more popular ones have been very stable since their inception.