The only one I'd really recommend is the debugger. An IDE is really an editor with a load of other gubbins added on, but if you can compile by typing make (or up arrow + enter) in a command prompt, then you don't need an IDE. If you can commit to SCM by right clicking in explorer and choosing the right menu item, you don't need an IDE.

Now I know some people need stuff like refactoring support (write your code right the first time :) ) or some integrated GUI designer (but even then, using Visual Studio I use Expression to do my GUI work, not the crappy XAML support in VS), and many people need intellisense and autocomplete (especially for verbose languages like Java and C# that have godallmightlylong names).

But for me, the GUI debugger is the only really good reason to use the IDE. I still use a 'command-line' debugger (well, windbg) but for day-to-day, its the builtin one to VS.