Actually, that's a feature... Ctrl-C exits insert mode. If it quit vim when in Normal mode, you'd accidentally quit Vim all the time. Less annoying to just show a helpful message in case you actually meant quit, than dump everyone who knows what Ctrl-C does back to a shell.

I think more important than exiting insert mode is (as I described in a peer comment, hopefully clearly) killing foreground processes you've spawned out of vim. 1) With exiting insert mode there's the option of using escape instead (or capslock or jk or whatever people bind it to), and 2), I think most importantly, with killing processes there's an actual race condition: if the command finishes after I decide to kill it and before vim registers my Ctrl-c, exiting is totally the wrong thing to do.

I would think that it's because ViM wants you to learn something about how the it works.

If it just exited with Ctrl-C, you'd never get to know about the : magic.

Not to mention Ctrl-C is ambiguous. What to do: Exit and save? Exit without saving? What if it's a new file? And probably others I'm not thinking about right now.

Much like Python

    Python 2.7.9 (default, Mar  1 2015, 12:57:24) 
    [GCC 4.9.2] on linux2
    Type "help", "copyright", "credits" or "license" for  more information.
    >>> quit
    Use quit() or Ctrl-D (i.e. EOF) to exit
    >>>

thats because you don't want to be using a variable quit and have it suddenly exit.

Definitely a feature not a bug.

    :'a,'b!longrunningcommand

oh, wait, that was not the right thing...

    C-<longrunningcommand finishes>c