How to display all values of List (text) instead of just the selected ones?
Even if no one here says they have needed to do that you can pretty much bet others out there have, so it's good that you have your code on drupal.org where other can use it if they need it.

I had a quick look over your code and your solution is the way I would do it: create a new field formatter for text list fields using hook_field_formatter_info() and hook_field_formatter_view().

A couple of things I notice though are:


The name of the module is very long. If you can make it shorter while still being somewhat descriptive I think that would be good (also better for you as the coder).
The markup you are outputting is not themeable, so if other people start to use the module and want to override the markup in any way it is harder for them. See image_field_formatter_view() as an example of creating a render array with a specific theme function. Also see hook_theme() for defining your own theme functions.
Text that gets displayed to the user that is hard coded into the module should be run through the t() function so that others can write translations for your module. See the drupal 6 version of that t() api docs page for better and more thorough information. t() is not currently used is for $form['instructions'].
This one is a bit more of a personal choice but I would probably make the  markup options a lot less than 500 characters. I would also quite possibly not allow HTML markup and just allow them to enter characters that would then display in a span. If the user wants something fancy they can override the theme function (as per point 2). Also, the instructions encourage users to use inline styles, which are not recommended. Users should use the classes you provide to add any css in their theme's css files.