It was an interesting challenge. So I decided to build every high level use case as a mobile application, even though there are no plans to release a mobile version of the application in the first release. It was the most interesting exercise I did this year.
1. Identifying the use cases became simpler
Thinking mobile first, helped me identify the key mental models I need to communicate to the product team. All I had to do was to wonder what a person will think about doing while he or she is walking down the corridor. For example, "Oh! I need to approve that expense report." or "I need to attend that training course". For more on mental models, the book by Indy Young is a great read. Mental Models: Aligning Design Strategy with Human Behavior
2. I was able to focus on the features that are absolutely essential.
Since the real estate available for me was limited, I was forced to think about the first thing a person needs to see when she launches the app. I was also able to focus on the main thing that the person must accomplish while in the app. Then I focused on the other 2-3 things the person can do while at the app. The limited real estate forced me to cut out everything that is not absolutely essential.
3. I was able to build small clickable prototypes in daily sprints
Since I was thinking about one problem and one problem only, I was able to sketch out the solution, discuss that with colleagues and build a prototype in one day.
4. The UI Design Conventions of the mobile client gave me a standard UI design framework.
I am not a User Interface designer. However, since I was using the UI framework provided by the mobile device, I was able to convey my ideas and thoughts without making any major interface design errors.
5. I avoided stepping on the toes of my User Interface Design colleagues
Since I was only building mobile prototypes, my UI design colleagues knew my intentions and understood that I was conveying a mental model rather than making an interface design suggestion.
6. It forced me to think about the details and identify gaps in my thinking
Because I had to build every screen of the prototype, I was forced to think about the steps involved in the process. I could not cheat, overlook or get lazy with my design. I had to take a stand, define it and be preparde to defend it.
I used Axure to create the prototypes and the mobile templates provided by Axure.
I recommend you give it a try. Thinking mobile-first may be a lot of work. But I am sure you will have fun. By the way, this is a not a very novel concept. Some pioneering companies are already doing this.