Firstly, emacs is not a quick-start environment. If I want to get started in a new language, let's say Java, downloading Eclipse will make getting started with Java insanely easy. The same with C++ and Visual Studio. Compare with emacs, in which I have to spend time getting things to work. I'm not saying it's worse in the long run (although I suspect you can't beat Java on Eclipse), but the overhead every time I want to try something new is large.
But the biggest problem with emacs is the "project management" functionality. Emacs is designed to be "just an editor". To do the most simple thing, which is to define a project consisting of a few directories, and be able to move between those files, is very hard. Pick up many modern editors, and you can, with one command, point to a directory and have a project tree, a "jump to file", etc, all built for you. Not to mention the way they create makefiles automatically, do code-completion, create tag files so you can jump to a function by name, etc.
I can just list the number of things I didn't have to know about before starting emacs: makefiles, tag files, etc. Not having to worry about these kinds of things is the biggest feature emacs is lacking.
Makefiles are an acceptable barrier to entry. I've lost a lot of hair trying to figure out how to build huge Java projects that the developers just built inside their five-major-versions-old IDE. ("Oh, just build it by clicking the 'export to WAR' button and then copy the file to each production server." No.)
Considering how trivial it is to write an ant file to do the same thing, it's appalling that the IDEs don't just do this by default. How can you write a good application if you don't even know how to compile it?
But the biggest problem with Emacs is the "project management" functionality. Pick up many modern editors, and you can, with one command, point to a directory and have a project tree, a "jump to file", etc, all built for you.
Of course, Emacs' version is a lot more flexible. The user can hook it, so he gets a fresh project REPL the first time he visits a new project. Or, he can hook the eproject-after-save hook, and commit the changes to a "backup" git branch, so that he never has to worry about not being able to undo.
The idea behind Emacs is that it can do things that nobody else has ever done before, and it continually lives up to this expectation.
(Also, "this is too hard for beginners". OK. Then how did I learn Emacs?)
It's not that Emacs or vi are lacking the features of an IDE, is that they were not designed to be one. If you want to make vim or Emacs an IDE you can, but it is not easy.
But the biggest problem with emacs is the "project management" functionality. Emacs is designed to be "just an editor". To do the most simple thing, which is to define a project consisting of a few directories, and be able to move between those files, is very hard. Pick up many modern editors, and you can, with one command, point to a directory and have a project tree, a "jump to file", etc, all built for you. Not to mention the way they create makefiles automatically, do code-completion, create tag files so you can jump to a function by name, etc.
I can just list the number of things I didn't have to know about before starting emacs: makefiles, tag files, etc. Not having to worry about these kinds of things is the biggest feature emacs is lacking.