One thing that has been getting on my nerves ever since I started working with Unity has been MonoDevelop. While it is a good IDE for the most part it suffered from a number of annoying issues, such as folds randomly opening, poor performance, and auto completion randomly not working. Your mileage may vary, and for the most parts it was doing its job, but the annoying hiccups kept happening all the time.
I tried out the official Xamarin Studio, but that one didn't fare better and on top of that always threw up error messages when opening a file. I was looking into alternate editors, but they didn't offer the rich C#-focused feature set of MonoDevelop, so I was stuck with it.
Eventually I was so fed up, I decided to go all the way back: no fancy IDEs, no GUIs, back to to bare basics: Vim. The cool thing about Vim is that's it's a very simple barebones editor that can be extended and customised to your liking. Vanilla Vim is nice, but you have not really used Vim until you have tailored every aspect to your personal liking. That's a gradual process that will take years of experience, but I do already feel very comfortable outside of the hand-holding restrictions of the IDE.
For an ideal Unity setup you will want a GUI client of Vim, such as MacVim. That doesn't mean that Vim will get all those fancy buttons and menus, although you can enable those if you want to, but the main advantage is that it can be launched like any other application instead of through the terminal. MacVim has also better mouse support (it's faster to resize windows with the mouse) and the character cursor looks different in insert mode.
The next thing you want is support for .Net and C# features. The plugin for that is Omnisharp:
https://github.com/OmniSharp/omnisharp-vim
Omnisharp will add pretty much all features that you have come to love from MonoDevelop, without the headache. Omnisharp also provides an interface for other plugins, for example you can use it with YouCompleteMe to get automatic code-completion:
https://github.com/Valloric/YouCompleteMe
There is a ton of other useful plugins out there as well and some are listed on Omnisharp's page. Another advantage of Vim is that it's not restricted to one particular language or framework. You can easily write your essay or design your HTML web page in it. Customisations can be set for each file type differently or you can use them over multiple types. Since it's all one editor you don't have to learn a new IDE for every project.
In fact, I have been designing my new upcoming website in Vim as well, the same editor I use for writing Grid Framework. How cool is that? Here is a video I found on YouTube of someone using Vim to edit text with the speed of though:
