Currently per the article states, the Emacs GUI uses a lot of hacks to display like a terminal. Because of this a lot of things like real smooth scrolling doesn't work like most text editors. Highlighting text with a mouse for example just doesn't feel right. I know people will say who uses a mouse to operate Emacs anyway but there are plenty of other scenarios that makes Emacs feel somewhat ancient, which it doesn't have to be. I don't use Linux DE a lot to know all of the quirks but on OS X when Emacs go full screen there are still black borders, etc. Imagine the possibilities of having a non-text only modeline for example, the ability to render pdf and modern JS/CSS websites, etc.

In what way does highlighting with the mouse not feel right?

Seems to work pretty much the same as in Visual Studio to me. Mind you I hardly ever use it, I prefer to mark the region instead.

If you can add what you want to Emacs without making the new features compulsory and without removing the existing ones then by all means go for it. But I suspect that the lack of some features is simply down to no one with enough enthusiasm and expertise to build them actually wanting them. Most of Emacs' features were built by people who wanted to use them so a lot of stuff ends up being worked on just until it is good enough for the creator's own purposes.