example?
A web-browser inside your editor?
A mail client inside your editor?
The ability to make HTTP-requests, parse their results, and format the results inside your existing editor window?
Editor? I do not think that word means what you think it means!
Emacs is an abstraction layer for a non-preemptively threaded virtual machine, one application of which involves editing text, sound, video, images, etc.
You forgot the web server :-) http://emacswiki.org/emacs/Elnode
ST2's plugin interface may be nice and sufficient for most "normal" tasks, but there plenty of things you simply cannot do.