Probably I’m much too late with my demand / suggestion.
But if there was one thing which I really loved in Half-Life 1 compared to Half-Life 2 was the more creepy appearence of the zombies.
In Half-Life 2 the Zombies just looked clumsy. Headcrabs just appeared oversized and the tottering style of movement in combination with the
moaning rather made them looks like some drunk fools walking down the street. They appear so vulnerable that I sometimes even felt sorry for killing them.
On the other hand the Zombies in Half-Life 1 had something more threatening. They didn’t appear like some sort human lifeform anymore, but more like something more gross, somehow letting you feel as player that these bodies aren’t in control of humans anymore. Their stiff and upright style of walking was just more creepy and they appeared much. Especially when you were trapped on a narrow catwalk and these things kept on walking straight into your direction. While you were firing your Berreta and started to run out of bullets, preparing yourself for close combat with the crowbar.