Halo 3 had some fantastic vehicle action that a lot of other games weren't doing at the time. I also loved the concept of a "mobile base" like the Elephant (as goofy as it was) and the Scarabs (even if they're just a single-player thing). Great physics too. I always wished games expanded on ideas like that as technology became stronger but they never really did. Games today are so stiff I'm not even sure they're capable of doing what Halo 3 did 18 years ago.