>>719053127 (OP)
Halo 1's campaign is very edenic. You discover a lot of biomes and destroy them. Going through forests, tundras, and valleys, your objectives are pretty much to find more land and map it out.
Halo 2's campaign actually feels like a sci-fi military shooter. The objectives are much more opinionated: keep this frigate safe, repel the attack, route the streets of invasion, blow up the alien capital. The settings are also more militaristic, from the stratosphere, to the African streets and all the way up to Delta Halo and back.
Halo 3's campaign is the most rustic and humane feeling. Most of it feels complete because Halo 1 and 2 made their campaigns around important aspects of recon and soldiering, so now in three you do both objectives at once, exploring as much as you can while doing meaningful shit like protecting people and routing enemies from friendly neighborhoods.