I am trying to figure out what it will take to convert an existing app to be compatible with Salesforce1. All there really is online is that you need to hit the checkbox on the edit screen of the VF page. What else needs to be considered to make this a user friend Salesforce1 app?
It depends on your customer requirement.
Lets say you want just mobile look and feel then the approach would be to just style your existing page with CSS friendlier to mobile.You may use plugins like jquery mobile ,polymer,bootsrap .
There is a great slideshow on slideshare.net which shows some of the best practices for how and when to use Visualforce in Salesforce1. It's not that easy and often requires a huge amount of additional development time.