The protocols were generally understandable (MSN for a long time was largely all MIME-based messages, and could be understood by watching a traffic stream)
Microsoft even submitted an early draft of MSNP to the IETF. I'd consider (at least the early versions) MSNP the next step up in complexity from IRC, i.e. you can write a working client with both presence and messaging within a day or two. This is from experience of having written a client myself, which I used all the way up to them shutting down the servers.
Used simple usernames/passwords for authentication (just fewer hoops to jump through)
Earliest ones were literally plain user/pass, then followed challenge-response, and later MSNP used something surprisingly similar to OAuth, where you authenticate out-of-band to get a token that is then submitted to the protocol server itself.