I must say I was rather surprised you hadnt heard of the original Half Life but after going through the posts and seeing your age that makes more sense as how young you would have been when it first came out. A big thing that made it such a great game for me and several other people was at the time it helped pioneer several things that are standerd in the game industry. It may have not been the first to do them but in my opinion it was the first one to do them together well.
One of the first things that was awesome for me was the fact that Gordan Freeman was a character alot more people could Identify with then other FPS heroes. at the time most shooters the character were soldiers, special agents, and other folks trained for combat. Gordan is just a guy doing grunt work at an advanced research facility, (he starts off getting into a hev suit and pushes a cart around, so grunt work) alot more people can identify with worries about being late for work at a new job He is not expecting trouble of the sort that happens. When the crap hits the fan his first instinct is what most of us probably would have done, try to get the heck out of there and possibly get help. Other FPS at the time had everything already gone to hell and your job was to clean it up.
The next thing is how the levels were laid out, at the time not many games had the design for thier levels that were directly contiguous with each other. Half life for me felt like a very real world as you traveled through it yourself the whole way and the differences in the environments felt natural and had a believable transition between them that you could probably see in real life if black mesa actually existed.
Gameplay was great in the fact that it had excellent use of weapons, enemies and environment all came into play, there were darn good physics for a game of that year. It did suspense and tension very well in my opinion with wondering what was going to come around that next corner, and having to use believeable elements of the labs to beat certain enemies rocked hard.
The story was nothing to laugh at either, or how it was presented. at the time alot of games still told story through intro screens, cutscenes and direct player interaction with NPC's, and while half life does have a little of what I mentioned above, most of it was told in a way that felt like you really were there, stumbling across people, evesdropping on Marines, running like a mad man away from the bigger monsters you couldnt kill via normal means, and not to mention the decent fights of marines vs grunts that you occasionaly stumbled on.
Half Life 2 is a great game of itself and took alot of elements from the original and while HL1 is not required to play HL2 I firmly believe that playing and beating HL1 makes 2 a much richer experience.
................................... of course after wrighting all that i realize i probaly should have just recomended going to a review site like metacritic and reading the reviews on the game as it had close to 50 game of the year awards in 98.