Comment by christop

In addition to logging as mentioned, I find that file transfers (encrypted!) that Just Work between users regardless of the network/client/server used seems to be missing.

I really like XMPP, and the extensibility it has is brilliant, but at the same time leads to a crazy level of fragmentation for the instant messaging use case. You're basically guaranteed that you can chat to others and maintain a buddy list, but anything much beyond that is a toss up, depending on the server used, how it's configured, and what client each user has.