I haven't even touched campaign or multiplayer yet, I'm just messing around in Forge.
Good god, Halo 2 Annie forge is awesome.
Hornet, Heretic Banshee, Gungoose, and Civvie Hog are all present (not the Spectre or the Shadow though, even though both were in campaign), the red energy sword is called the 'Infected Sword', new weapons include Silenced SMGs, 'Survivor' Magnums and Shotguns in addition to normal ones (not sure why, probably the way Infection spawns weapons going by the names), the UNSC have both removable and permanent MG turrets (the covvies only have the permanent plasma turret)
New 'snap to grid' feature is way, way better than H4's Magnets (which are still in it, for some reason), flattish rock areas, cliffs, trees, basically lots of nice nature stuff for natural maps, and all the structures are done in the Halo 3 sandstone style, as opposed to Reach's Forerunner and Halo 4's terrible plastic UNSC texture set.
Oh and the three new forge maps are all pure empty space. One's in what appears to be Impact's skybox (ie space) from Halo 4, one is the ocean (which is solid and not a kill barrier) like Forge Islands, and one is flying amongst the clouds over a Halo, which looks freaking sweet.
All in all its pretty neat.