So much, let me recount some of it.

There's a lot of invisible tweaks like the assault rifle has a smaller initial spread which makes it usable at medium range with short bursts. The needler fires faster. The warthog accelerates to top speed faster. Hunters no longer die to one magnum shot. Flood popcorn forms don't chain explode nearly as much and they do a little more damage to the player so you can't just totally ignore them anymore. The player can jump ever so slightly higher allowing them to reach certain areas during combat more easily. Vehicle damage has been tied to speed so tapping (or being tapped) by a slow vehicle isn't an insta-kill. Ghost and plasma turrets have tighter spreads, both when firing at the player and when the player is using them. Marines will now drive unoccupied vehicles and follow the player.

The energy sword, flame thrower, fuel rod gun, and sentinel beam gun are all usable in the campaign now.

More enemy types are added. This is especially noticeable with the flood which has elite-flood forms using shields now. There are zealot flood with swords. There are cloaked spec op flood. This makes the flood more interesting to fight and keeps plasma weapons important to the mix against them.

There are now ODSTs in addition to the normal marines. They have slightly more aggressive AI than the standard marines, so they are more active in combat but can also get killed faster if you ignore them. They replace marines where it makes sense in the campaign.

Enemy mixes have been tweaked throughout. The Library had work done to make the spawns less of a slog to get through by placing the additional spawn waves more heavily behind rather than in front of the player.

There are non-combat animals on the ring now. From butterflies to big grazing creatures.