Is it feasible to develop a full-blown web application with SharePoint using a text editor and command-line based environment? Say I wanted to use Vim, svn/git, and powershell. So minimal use of Visual Studio, SharePoint Designer, etc. Could I do it?
(The question is mainly whether it is possible, but the obvious corollary is whether it is sensible, so opinions welcome on that, too!)
You say "full blown web-application" but it depends what features you wanted. As Thomson above states, you can do normal C# coding in just a text editor and deploy your managed code, but page customisations can only be done through SharePoint Designer.
To answer your original question of whether or not it's feasible, yes but incredibly time-consuming. Without Visual Studio you have no IntelliSense, so you'd spend far too much sorting typos and reading up on the SDK for methods and properties which would normally show up automatically as you type.
How are we going to deal with customized pages. You need to have Sharepoint Designer to do. There is no other way.
Normal C# coding can be written in Text Editors and can be complied in C# and make use of the dll
Possible, yes.
Sensible, no.
Any particular reason why you might be interested in this approach? As a SharePoint developer you can live without SharePoint Designer - but Visual Studio is a must (if you want to be productive).
Fortunately, SharePoint gives us lot of out-of-box functionalities. You may use it in a better way to develop the applications. Like Views, Content editor,media library views etc...
