The hl engine is old yes, but I'm contiunually surprised by how good some parts of it still look. The crossbow in HL for example beats alot of weapons models in recent games, thats just sad. It's a compliment to HL, but still.
The bullet impact effects in CS are still great: -bullets leave a hole on both sides of a door, the smoke and particle effects give a good tactile feel to shooting, rounds make a different sound when hitting different materials wood/metal/concrete
I know you are saying why am I so impressed with that. Because some games today still get those basic things wrong. YOu would think they could take note of successful games and learn.