I like it.
I was fairly alarmed everytime the flood arrived in the first game, and if Bungie manage to make the elites genuinely hard and scary, that would mean something else than the original Master Chief tank gameplay. I haven't played ODST, but I guess that's different in that regard.
A change in atmosphere would only serve the series well, in my opinion. Combat Evolved had a masterful atmosphere, but the two next were slightly worse in that manner, so a different feel with the game would be awesome.