Technically speaking, you can modify just about everything in the program, and relatively easily at that. For example, to modify the main dropdown menu, go into Packages/Default/Main.sublime-menu and you'll find a JSON file that is human editable containing the entire menu structure.
In fact it's hard to find anything in Sublime that *isn't* configurable, and usually (relatively) easy to configure. "Easy" being a relative term, as thus far this is very much targeted as a developer-configurable program. When I say easy I mean the configuration is in plain text editable formats like JSON and XML and not in something compiled or encoded requiring reverse engineering.
It's better to view Sublime (as it is so far) more as either a developer targeted environment or as an amazing platform to build another editor on top of, where you might build out more GUI components to change settings so it's targeted at less savvy people or people with less time to spend programming their programming environment, and then ship that as "MyFancyEditor powered by Sublime" kind of like how Eclipse is the foundation for a lot of things built on top of it.
I'll say from the perspective of having pored over the source code from top to bottom, and working on tightly integrated components, Sublime is an amazing platform at core and has a tremendous future ahead of it. But it absolutely is very much the internal skeleton that doesn't have flesh yet. But keep in mind that you're not required to purchase a license as of current. I would say that it is close enough to being a base level of "complete" that I agree with the decision to start prodding people to get a license, but the time hasn't come where you absolutely have to.
I will also say that the $60 is worth it in my experience based on what I've seen in other editors. While it may not be quite visible in using the program from a user's perspective, at a platform/API/meta-framework level Sublime is starting to provide something well beyond the other options that are there. It's extremely powerful while also being way more accessible (at the core level).
I would say that the time isn't quite right to be using Sublime unless you're both capable of programming it and then willing to put in to make it do whatever extra things you want it to. It's getting there, but it's not quite there.