Never mind the graphics or age, you can't stop coming back...
Anyone who knows their FPS stuff should know what a milestone this was, and in many ways is.
From the start you're involved. The often complete lack of pointers leaves you totally immersed, and clueless as to what the correct thing to do is. All you know is that there is a way through, and sometimes even this isn't correct.
Sticking to this also makes it a somewhat uncomplicated idea, done to perfection. If it hurts you, shoot it. If you're not being hurt, find a way around it. The levels blend seamlessly, despite each having a distinct tone. The difficulty is what you make of it, both through being adjustable and that if you waste your ammo, you're creating a pickle for yourself later. On the subject of ammo, each weapon is unique enough to add a new facet, and overall they offer a diverse range, even if you do end up coming back to the one with the most ammo provided.
I'm avoiding mentioning specifics, because frankly, you shouldn't be reading this, you should be playing that.
Having said all this, after playing it, get Black Mesa. It's great to see the original so brilliantly remastered, however the original should definitely come first.