It seems, even though we talk about Telepathy a lot, that there are still people out there who believe that Telepathy is really just a reimplementation of libpurple, just with an annoyingly retentive specification, and D-Bus.
The big thing to understand about Telepathy is not that it's platform independent (even though it is, and that's pretty neat). Or that it's modular (even though it is, and that's also pretty neat). The neat thing about Telepathy is that it's a service. Telepathy provides communications as a service.
What does this mean? This means that Telepathy is not just a library for enabling communications in Empathy, or Kopete. Telepathy can enable communications in everything. Any program can listen to and interact with Telepathy. This means that you can send a user files, straight from Nautilus; or share your desktop with Vinagre/Vino, via a Telepathy Tube; all without having to set up your accounts in each of these programs (this information is stored in the Account Manager, a session-wide Telepathy service responsible for maintaining accounts and connections — Empathy's accounts dialog is just a user interface to this service) or needing to establish a connection per application.
Because the information you need is available everywhere, this allows communications features to be integrated into places where they make the most sense in your desktop, rather than implemented in your contact roster (like another chat client does). For instance, mail notifications that Telepathy learns about from webmail services such as MSN and Yahoo (note: not yet implemented) could be plugged into the existing mail-notification applet, or into Evolution to hint when to pull from IMAP. GNOME Shell could provide an embedded GMail-like chat-UI, with popout chat-windows provided by Empathy. All of this is possible without those applications having to have their own preferences dialog for your accounts.
telepathy-glib provides classes for talking to the the Account Manager and Channel Dispatcher, setting up channels and handling contacts. In the future this will be expanded to make it much easier to develop Tube clients and other common tasks (note: avoid libtelepathy and libmission-control, they are deprecated traps and not to be used). Hopefully soon there will be a telepathy-gtk to provide common, reusable GTK+ widgets that applications can use. Telepathy-Qt4 has made it's first shared-library pre-release for those who prefer the other toolkit. If you're interested, I've been working on the Telepathy Developer's Manual.