I just played through Surface Tension in Half-Life and I think the biggest problem with the parts that were cut for this mod isn’t so much that they are amazingly fun or memorable, but that they really are a big part of the storyline.
The part with the mounted gun where you have to gun down at least fifteen vortigaunts teleporting in is followed by a room with a HECU body strung up by rebar on the ceiling. You use one of the alien jump pads to get up on a ledge of a severely destroyed building and witness two more HECU being pinned down by a vort and an alien grunt.
After this is the snark section where you drop down from the vent and kill the guys in the garage. You can tell the marines are getting jumpy at this point by them shooting up the vents just from hearing something. Then there’s a few on a cannon that you have to kill and use the cannon to break a hole in the wall where there are two more alien grunts. You get the feeling at this point that the aliens are literally everywhere and the HECU are starting to lose.
The next room is a giant battle between the HECU and alien grunts and a few vortiguants where a tank busts through the wall with marines coming out of it. The HECU loses this battle and after you cleanup the very next hallway has an alien grunt tossing a marine through a concrete wall.
After that you drop down on two marines hiding in a mostly destroyed building from an alien grunt and a few vortigaunts. There’s yet another alien grunt way up on the roof of that building. You use the jump pad to go up and drop inside and go into the pipe that a marine throws a satchel charge into. After killing him the next room is a bunch of HECU running around like crazy trying to avoid multiple headcrabs.
By the way there’s alien terrain scattered throughout the buildings in this chapter. From multiple defunct jump pads to webbing in the corners etc. I like to think of it as the creep from starcraft. The whole area is clearly being taken over by the aliens.
After getting the guard to open the door to the garage area the gargantua completely gibs two marines against a wall by pushing a vehicle into them.
Needless to say the HECU is getting slaughtered at this point. There are a bunch of small numbers of them cut off at every turn by aliens. The entire point of the last half of Surface Tension is to show how badly the HECU are losing at this part of the story. This is why the cut parts of the map are so important as Forget about Freeman is the next chapter.