Proprietary Nature of Most IMs a Major Issue
The biggest problem with the many existing instant messaging platforms is their proprietary nature. Users of one system cannot communicate with those of another system. In the end, users are forced to run multiple clients on their machine and keep track of who is on which service. Also, the providers of these proprietary services rarely develop clients for any but the most popular operating systems. Linux users, for instance, don't have a vendor-supplied client for AIM, MSN, or Yahoo. Instead, developers must develop clients for these unsupported platforms (such as GAIM or GICU), often with vastly reduced functionality.
As far as platform support goes, there are Jabber clients for most any platform out there. Several client packages are available for Linux and Windows environments, and there is one for Macintosh--even a client for the Newton MessagePad! In its current state, Jabber allows users to connect to servers and exchange messages with other Jabber users. Various Jabber clients (such as Gabber and Jarl for Linux, and WinJab and JabberIM for the Windows platform) offer different features, and each is at a different stage of development.