Compose-region is like an alias. (Yes i did mean replace) the editor displays the unicode character instead of the text. This can be used for a lot of different things.
Some of the functional languages (for instance) have modes that display a lambda chatacter instead of the lambda keyword. Some extensions go really all out. prettify buffer for instace offers a programming experience with some pretty fantastic symbol replacement. I have enjoyed it greatly. Some latex and other doc writing modes use it for displaying formulas in a more readable (friendly) fashion.
In my case i would like it for a set of orgmode plugins i am working on. One feature to start (org bullets) that can pretty up an orgmode buffer with nice looking (and different) bullets for each level of indentation. It is a very nice extension.
IMHO sublime is a superior application, its performance, even on windows platforms, is what draws me to it, its just missing org mode and the org related tooling. I figured i would work to extend the existing orgmode plugin i found on package control. I have a capture feature and refile functional and am looking into some of the navigation and ease of use bindings but am sorely missing some of the minor visual pizazz of extensions like org bullets. I figured it wouldnt hurt to ask for a means of implementing it. There may be other ways to achieve this, i just cant think of anything that wouldnt tank on a larger file (performance wise)