Named Game of the Year by over 50 publications, Valve's début title blends action and adventure with award-winning technology to create a frighteningly realistic world where players must think to survive. Also includes an exciting multiplayer mode that allows you to play against friends and enemies around the world.
What Half-Life does well is tell a story through action
HL has no cutscenes because cutscenes are the opposite of action, they are inaction because it takes control away from the player.
One of the key features of a "Rail Shooter", go from point a to point b with few diversions & no backtracking.
Rail Shooter as in like a train, you always keep moving.
Valve knows that backtracking gets frustrating faster than anything else, and is the most artificial way to enhance difficulty (hell increasing the damage the player takes is a better way if only marginally).
Valve is also fond of vent crawling and puzzles which I've never seen much appeal in apart from the initial novelty of vent crawling.
Apart from being very moddable and userfriendly,
Half-Life also does something incredibly well that has helped modders improve their own skills, and thats level design. Each place in the game looks hand crafted. No copy and pasting of sections or randomization here! It also knows how to be challenging without throwing tons of enemies at you, and thats with good enemy placement as well as knowing when to give the player rewards (weapon/ammo/health placement).
Its a game designer's poster child, thats what it is.