Yeah, there's one big hubworld that you explore and then go into flashbacks as you find clues to the story.
So structurally it's different (hub, flashbacks), tone is different (smaller scope, less colorful), perspective/narrative is different (you play different ODST in one city), gameplay is somewhat different (no duel wielding, health packs, weaker character, squads, no BR and AR, visor mode, etc), Firefight (horde with vehicles and skulls)
Graphics are pretty much the same and that's what most distinguished mentally-challenged fellows look at so they think it's a rehash.