I love the changes and I wanted to make some kind of record of the ones I found, and hopefully for people to add ones I missed.

Gonna exclude changes that are just the removal of load zones.

Trainstation:
Gate in front of vortigaunt is openable. Has stairs to a walkway.
Gate by the stairs after you jump out barney's window is openable.
Door next to alyx's elevator is openable.
Elevator next to Kleiner's vending machine is usable.
There's a garage across the street from Kleiner with usable doors, but I don't know how to get to it without noclipping.

Canals: No changes

Water Hazard: No changes until the building where you get the 357. Here, in the blue warehouse where you jump across the shipping crates, the back garage door opens into a large open area, with another openable garage door on a building there. Then, after the doorway blocked by crates, there's a door next to the metrocop that can open.

Black Mesa East:
The gate next to the elevator Mossman takes you down opens. You can jump on the truck to get another route to Eli's lab. The hallways connected to Eli's lab are more interconnected and there's a new generator room.
Surprisingly, the vortigaunt kitchen is still not accessible.
A garage is Dog's yard is now openable.

Ravenholm:
Grigori's balcony can be accessed.
The door to the electrical switch can be opened.
There's a new area next to the falling car trap.

Highway 17:
The parking lot after where you get the AR2, the garages there open.
On the dock where the crane picks up the buggy is a gate that lets you access a large open area and a bunch of warehouses.
The crane complex after NLO: The two small buildings by the thumper can be accessed.
After the crane puzzle is a warehouse. After that warehouse is a ramp. Normally you turn right here to keep going, but the gate to the left opens and the warehouse behind it can be accessed.
After you jump through the window, the buildings can be entered.

Will update post with new info.