[eluser]theprodigy[/eluser]
Besides documentation and community (which are actually tied for number 1 in my opinion), I like the fact that I don't have to open my command line terminal just to create another Controller, Model, Library, etc. I can just right-click, choose New File, and be done with it. (Of course, you could always use command line to create them if you want: vi my_new_controller.php ;-) )
Plus another thing of note is that if you are used to using a particular library in your development, CI lets you drop it in and use it. You are not forced to use their pre-made classes if you don't want to (with certain exceptions: Controller, Model). If you have an email class you enjoy using, use it. You don't have to use CI's version.