I'm a recent convert to Jabberness - a total believer. Jabber's a fantastic system but it's getting bogged down not by its complexity but by its analogy (that and crappy clients).
To me, the principal problem is that unlike the other IM systems out there, a Jabber ID looks exactly like an email address. This is great from a geeky theoretical standpoint because it's organized and "standard", but honestly, it's COMPLETELY confusing to most people, and even to us techies since it took me a WHILE to really Get It.
Think about it this way. There are zillions of email servers out there and when you tell someone your email address, the person who's waiting to jot it down is ready for just about anything. Because we know how emails addesses work. It's your name or funny unique id "johnjo222" with the at sign @ and a server name which could be ANYTHING dot anything. Even my Mom understands what's going on when she attaches to an email server to get her email. She never thinks that she has to connect to your email server in order to send you email, but with Jabber that's a common misconception.
Instant Messaging just isn't like this. If you're using one of the big three IMs, it's just an application that Just Works. There are no server settings and normally no @ signs. Granted with MSN you can use any Passport name now (which is what I do), but for most people, "@hotmail.com" is the ending that signifies a MSN Messenger username.
So the Jabber system is this fantastic open system that could allow true innovation in the messaging and presence space if it was used widely, but adds a layer onto messaging that none of us are used to! You have to have a server and your ID looks just like an email, but (99.99% of the time) isn't. This is too confusing. I mean, from a technical perspective it works great! Just like any two email servers on Earth can send email back and forth, with Jabber, any two messaging servers can send messages back and forth. The problem is that we've all been TRAINED to have this magical app that Just Works.
Random thought: You have to wonder why personal Instant Messaging wasn't dreamed up years ago and tacked onto internet standards like SMTP and POP3. I'm not talking IRC, I'm talking personal IM like we have today. Odd.
Anyways, my point to this rant is that Jabber needs a Passport-like service so people can think of Jabber as just another IM client like the rest. You'd sign up for a Jabber account and enter in your current email and password and be done. Then the Jabber clients would look to that big repository in the sky - it could be centralized or it could be distributed like Gnutella or something - and find your server details for you to log in.
I'm not saying take the flexbility of Jabber out of the system by centralizing log-in, I'm saying the system needs to allow user names and emails live in better harmony. Honestly, the whole server nonsense is confusing. I remember it was JUST recently I grokked the idea that ids on two servers can talk. It's been obvious to me for a dozen years that my email message can find you regardless of which server you use, but IM? My brain just fried: On different servers? Huh? Do I need to log into your server in order to send you a message? How does this work? (Answer: It works, great, it just works like email does, not like IM does, which is the problem.)
I guess this could just be an education thing, but that's an uphill battle.