As I've read various Jabber books and specs over the past few days I have come across a few gems that have glistened "next ubiquitous feature," in so many words.
Offline messages - Yahoo! and ICQ have done this for quite a while, but it never seemed to catch on with AOL. With this feature, a message is delivered to its recipient even if they are not online when the message is received. If they are offline, the message put into a message queue, tagged with an "originally sent" date element, and delivered when the recipient is available.
"Displayed" notices - "your buddy is typing a message" lurkers can breathe a sigh of releif, as they no longer have to anticipate whether the other party saw their message. When the displayed element is included with a message, the remote client will send a response back when it believes the other party has displayed the message. This would likely be when the other person clicks/brings the message into focus. Being a component of message, these are compatible with offline messages in both directions.
Message expiration - Jabber lets a message expire after a set amount of time. If the client were to implement the expiration policy, it would destroy a message at the correct time. Likewise, servers support this feature with offline messages and do not deliver expired messages.
Suspend / Resume - This is unique to HTTP bound transport layers (XEP-0124), but is a nice feature that Meebo doesn't currently support. In short, it lets a user leave a site and resume their session upon return (obviously client UI state will be lost). From my understanding, the server queues messages on the remote end as it would with an Offline message and delivers them upon a Resume command. This behaviour would likely be handled by the plug-in rather than the server itself, since they appear "available" between suspend and resume. There is no upper bound as to how long a connection can be suspended.
While none of these features are extraordinarily killer, I think that once discovered they will become ubiquitous.