As I just finished watching the directors cut I can tell you that there are no noticeable differences in plot. All the extra footage only adds about 5 minutes to the film and in all honesty I can see why it was cut from the theatrical version. That being said what was cut were some small character moments. These moments help develop the character of Steve the most and expands the friendship that builds between Andrew and Steve. We also get to learn a little bit more about Steve and Matt in an extended diner sequence that has a few fun bits (but I can see why a lot of it was cut down as the whole sequence runs about 2 minutes longer than the same scene in the theatrical cut. Beyond that it's mostly minor insubstantial stuff that was added in that shows off the powers a little bit more.
As far as the ending it is the same as the theatrical cut.