Some do seem unrealistic, but with a facility as advanced as Black Mesa, you can imagine they're resources for their weapon research department. I am not going to spoil the weapons for you, I'm sorry. But as you find them and learn to use them on your own you will enjoy it much more. I think that naming off and describing the weapons would ruin much of the surprise. Interaction with characters and objects is like no other game before.
Enemies are also fun to watch.
On on occasion I came across 2 aliens fighting over a dead scientist's body for lunch. Well, my shotgun is about to make them today's lunch. You do not observe these events, you are a part and usually the center of them. Well either decision will affect your outcome in some way. Half-Life's multiplayer aspect was not heavily concentrated on according to Valve.