Only campaign opinion comming up since I gave up on MP very quickly after I saw I couldn't pick SWAT or other specific modes.
Yes, Explosions, technical issues, flat lightning etc. this all exists and affects the look and tone of the game but I think there are 3 things that have the biggest impact and you named them partially.
1. Open World.
Open world was supposed to grab that "omg it's huuuge!" moment of Halo 1s second mission where you crash on Halo and try to evac all the Marines that drop pod onto the ring like you.
Back then, one was amazed and it had the feeling of a open world level because you could tackle most of those evac by coming in from different directions.
Halo Infinite feels like it tried to take this and turn it up to 200%, but at the same time failed horribly. It has the same issue most open world games has, with some even bigger flaws because of it being a shooter.
It lacks "content". Not in the terms of side missions or anything, but in "why should I drive through the world and engage in fights etc".
The enemie groups are usually tiny and you can clear them pretty easy even on legendary. Sometimes it even feels like you clear them "by accident" when you run half of that group over with a warthog because you simply didn't see them. They don't pose any threat, so why should I engage them? Once you collected the audio logs etc. you don't even have a reason to wander the world anymore.
It lacks dynamic events, and I don't mean those random drop ships appearing. Something more to Marine vs Banished fights etc. Maybe even Banished trying to recapture FOBs etc.
It also gives no sense of progression. Yes, you free Marines and get points for that and FOBs are your Mobile Spawns but.. the game world doesn't reflect any of that. You could play through the game never freeing Marines, never assassinating HVTs and not freeing any further FOBs and the game would be exactly in the same state as if you literally decimated every banished 3x on the map. If clearing Banished facilities would weaken the force, or freeing marines would grow UNSC presence in those sectors with increasing amount of events where UNSC attacks Banished patrols etc. then you'd feel like you make a difference. Currently? Not at all so why bother except for the achievement.
2. Weapons
The Weapons were always something very iconic to Halo because of its closed level design. Power weapons were scarse but really packed a punch. It felt great using the Sniper, the Rocket launcher or the Spartan Laser. You felt like this unstopable force that was like a armored John Wick as you plowed through enemies with first UNSC weapons, then you use their Weapons against them but also being very efficient because if you run out of ammo and the only thing lying around are 20 plasma Pistols, you know you're in a tight spot. You had to make your shots count so you could swing from good weapon to good weapon with the occasional driple of power weapons. Sometimes they were tucked away somewhere on the map so you had to do a little bit of exploration, which brought the risk of not finding those weapons but still being forced into enemy engagements.
With the ability to simply restock on those power weapons, they lose out most of their power. It's just no fun if I can tackle every mission from a certain point with a Sniper, a rocket launcher and a Scorpion Tank. It doesn't even fit the story narrative where UNSC are supposed to have been hunted down after a grave and devestating defeat... but somehow one sole Pelican can fly around the map, dropping me a Scorpion whenever I see fit and we have the very best weapons just lying around on piles at every FOB..
3. Enemies
And I don't even mean regular ones. I actually liked that they added something like "classes" to the Brutes, with there being Snipers that all have distinctive looks or the Skimmers. I really enjoyed those, even though it feels like they are just a replacement for the Drones. Why they didn't just use the drones? I don't know.
I mean the Bosses. Healthbars for bosses is, for me, the single biggest issues I have when it comes to gameplay with Halo Infinite. The Story is a completely different issue, but why the healthbars.
I know that "technically" there were bosses before like Halo 2s Tartarus or Halo 3s 343 Guilty Spark. But the addition of having actual health bars displayed is just awefull in my opinion. It just demotivates you so immensely if you empty out a whole AR mag into a enemy and only see it take out like 2% of his health. Without healthbars, you'd have to constantly make the rational decision, do I want to empty out another mag while he charges me and potentially kill him, or do I try to evade and reposition him for another shot. Now, you just have to look up and see all the info. Also, IMHO, them scaling health AND shield was a mistake too.
I am completely convinced that if the Shield would be like the shield of normal elites, it would be already 40% more bearable, but spending so much ammo to chip away the shield only for them to retreat and recharge that shield in 5 seconds is so, so frustrating. On legendary, 99% of the things one shot you, while you do what feels like the damage of a rounding error between 0 and 1. Not much. They have turned the enemies from fearsome bosses into bullet sponges like every other game has them.