I think you've focused on the superficial elements only...
Halo had huge open outdoor levels
For a
console shooter of that era, sure, but I'm not sure to what extent they're actually a huge technical accomplishment. Polygon density is pretty low in the big exteriors, they tend to use simpler materials, and (aside from the usual "throw a bajillion alpha layers at the screen" spikes) the big exteriors are where the game's performance issues tend to come from.
without too many invisible walls (if any - people managed to get into almost everywhere in that game)
Oh, there are a
couple (I think it might literally be 2) death barriers, generally intended to prevent people from checkpointing at the bottom of an inescapable area. But there's a good chance you wouldn't even realize that it's a death barrier that killed you, hehe (they're at the bottom of falls that are just deep enough that you might think the fall killed you).
It supported a large number of quite intelligent(ly acting) enemies that could be surprised, scared, tricked, used against each other and so on.
The AI is a tremendous technical achievement, but I'm again not sure that it's a strong case of "pushing the hardware." Bungie got it to work so well and convincingly by making it extremely simple under the hood.
Still, at least it wasn't glitchy and they always got out of the driver's seat when necessary.
Heheh, more like "at least they didn't get into the driver's seat to begin with (except for ghosts)." In the later games, a marine trying to drive your warthog is often a marine who needs to be killed.
Halo is weird in that the allied driving AI feels like it consistently went downhill over time. Halo 1 thankfully didn't have much of it, Halo 2's is
okay at best, Halo 3's is worse, Reach's ensures that if the enemies don't get you, the cliff will...
It also had very cool ragdolls
I absolutely love Halo 1's body "physics," but there's no ragdolling system. Animations are entirely playback of a small set of hand-animated sequences, with exception to corpses drooping to align with the scenery after they come completely to rest (which is absolutely hilarious on account of being the sloppiest "IK" sort of thing ever implemented in a game).
Remember the Warthog grenade jump videos?
Yep, I was a small part of that community over much of its life. I made
a couple tricking videos myself.
Did you see
Tower to Tower? I'm the guy who recommended the use of Don't Stop Believing, because of
this 2005 video.
The system was pretty robust (very hard to get stuff stuck in walls and so) and produced some truly spectacular results. Nowadays we're used to all this dynamic stuff but there wasn't much of it before Halo.
Heh, I'm not sure "robust" is the word I'd use for it. Vehicles are the only objects that have "complete" bidirectional physics relationships with things around them, and their physics are basically assembled from a couple of locked-together inflated balls.
The physics are some of the most fun ever, but it's some extraordinarily bare-bones tacky stuff (and that's a big part of why it's so fun).
Also, multiplayer. Even in Halo 1 it was huge, the first time you could really play across multiple consoles.
That sort of networking model wasn't anything new for Bungie or the industry.
There were also a few smaller tricks like the light shafts through the trees
This is true, and I like how you specifically said "through the trees" because literally the first thing that everyone did after landing on the Halo is look at the trees and the sun through them.
or the plasma weapon stuff
They've got some nice use of the particle system, but I'm not sure what specifically you're referring to.
Or using IK to plant the characters' feet on slopes.
Halo 1 doesn't do that at all.