Summary
Wow.
The Good
This is truly one of the best games of all time. This game takes you on a journey, and never lets go until the end.
First of all, it involves you with the characters. Gordon Freeman is a real character, not just an action hero. He doesn't want to be involved in this - he simply does not want to die.
Secondly, this game actually scared me. It takes a lot to scare me, but the little barnacle-thingies-that-I-forget-the-name-of dropping onto my head spooked me.
Thirdly, the AI. The marines are some of the best opponents ever programmed. The scientists are generally quite smart.
Fourth, the bosses. These bosses are so badass looking and well programmed it amazes me.
Fifth, the graphics. Was the best-looking shooter when it came out next to Unreal.
Sixth, the episodes. I know it isn't an original idea, but I love being able to go back and forth between levels.
Seventh, the expandability. This game is the most expandable first-person shooter out there. Plenty of mods will be released.
Eighth, multiplayer. A whole lot of fun, especially Team Fortress, which flat-out rocks.
The Bad
It ends. It doesn't end too quickly, but it ends. Also, a few bugs here and there, but no more than average.
The Bottom Line
My personal favorite single player first person shooter, excluding System Shock, because that's a hybrid.