It's pretty obvious that Gordon is on good terms with most of the guards in Sector C. Even if he goofs around and slacks off despite being needed down in the test chamber, the guards will do nothing more than smirk at his antics, make some light jokes, and say they'll go get a beer after Gordon clocks out. Some may mention that they and a few others were thinking about some poker, and will keep Gordon posted; and one good-naturedly teases him: "Oh boy. Look sharp. Here comes the administrator. Haha! Gotcha! Aw man, I gotcha good!"
In the cafeteria you can find one guard nursing a cup of coffee and talking about him putting in overtime. Gordon seems to sympathise with him, as we get a genuine smile and a 'cheers' in return.
In surface tension, if you postpone putting on the Hivehand until you find a nearby security guard, he will usually joke or even give off impressed responses when you demonstrate how to put it on, as if the whole strange event was happening before the Resonance Cascade.
Fighting to keep Guards and Scientists alive. Due to improved AI, better voice acting, and variable appearance, they feel a whole lot more like people than they did back in Half-Life and because of that you will feel much more inclined to keep them alive. And you just generally feel pretty awesome when you do.
It helps that you can actually recruit more than two NPCs at a time now.