Great post by Paul Freet from Georgia Tech's Venture Lab about the "3rd product."  I like this approach as it helps frame vision vs. practical realities.  Essentially look at your big idea as the third product you release.  What is your first one going to do given constraints on time and resources and the need to generate real customer interest?  It is important to have a grand product vision but it is also important to know where to start to deliver meaningful value.  Sometimes you must crawl before you run and this is especially true for the first release in your product lifecycle.