Many (most?) site redesigns or migrations seem to be geared toward planting a flag, emphasizing the thrill of a new look or gee-whiz functionality rather than setting a site up for continued success.  Of course, many contracts with development firms will include some time for fixing bugs, but maintenance should really be set up for long-term success.  Key aspects of ongoing success of your site include setting up the system for easier maintenance, having product management to control quality (rather than bolting on functionality causing maintenance to be more difficult), and appropriately staffing / training.  Please also see the WelchmanPierpoint blog  for more on setting up your Web Operations.