I am going to give an answer that everyone else hasn't given. Everyone is partially right about edge to edge quickness... but that is nothing can't go read in the Ski or Skiing buyer's guide. The issue that a wider waist presents is that it is much farther from your boot, thus much more difficult to get it up on edge to begin with. Once it is on edge, it is usually difficult to keep it there - especially on hardpack. As uncle crud mentioned, there are other factors that come into play that may make or break a ski in various conditions. Rolling a wide waist from one edge to the other can be done just as fast as any ski out there, but the issue comes in that most skier's cannot actually get a waist over 85mm up on edge. If you are looking to learn to carve or are not proficient at carving, an 80mm waisted ski isn't going to teach you unless you already know how to force a ski on its edge and use the side cut. Once the ski becomes significantly wider than the base of your boot, you start to feel the difficulty of getting the ski on edge.
We could go on for hours about the pro's and con's of each waist width, but common sense will give you most of them... Your main goal should be to figure out what qualities are important to you, and then try a range of waist sizes to see how you react to each one. You will know when you have gone too wide and need to turn back...