This is interesting from other productivity apps (namely calendars and to do lists) that I've seen in the past. The witticism in your name reinforces the theme with the story, and the design for 'Mr.Habbit' is also unique. Design aside (which is nice, by the way), the interface is relatively easy to use and I've set a few habits for tomorrow.
edit: Found some areas of improvement, I'll add more to it if I can think of anything.
- The site (mainly images) take some time to load, and vary between three to five seconds. Still an OK waiting time -- not the best. More importantly, it takes some waiting time for logging in and out. The clock animation also lags a bit as it turns and zooms in. I like that it goes from sketchbook / drawing to cg art.
- There isn't an easy way to go to the "future self" section of the site other than manually typing or bookmarking "habbit.me/futureself". The other way is through clicking "Enter Time" at your account's dashboard. Might it be better to start right at the future self page (especially for logged in users), rather than starting at the dashboard every time?
That's pretty much all I can think of, if there's any more I'll add to this post. With a few improvements, I think that you definitely can go far and I wish you the best of luck.