I think Halo gets a lot of flak because while the first game had some innovations the sequels didn't add as much to the genre. But that makes sense because the sequels all take place immediately following the conclusion of the previous game. But by the time Halo 3 rolled around pretty much everyone had stuff like drivable vehicles, smooth indoor/outdoor transitions, limited weapon selection... a story.
But considering the serial nature of the series, I really don't think it's fair to knock halo for not instituting big changes with every subsequent game.