UNSC Marines have changed significantly. See the fine detail for the Halo Reach Marine version?  The new game engine has worked wonders!

Elite renders have become much more complex as well.  More geometry in Halo Reach than in Halo 3.

The evolution of the Grunt is readily apparent.  These annoying creatures have evolved a lot over the last 10 years.

The MA5 assault rifle has changed very little.  Over all the weapons have been impressive for all Halo games.  More geometry in Halo Reach also means more moving parts while firing.

The Warthog is the most famous Halo vehicle ever!  Expect to see improved versions of this as long as new Halo games are developed.  The basic overall design is the same with some variants.