@SympathyX:
I'm interested to hear then. What to you makes it an adventure game then?
Just to confirm in case "I'm interested" was sarcasm, that I mean no ill-intent and I'm not trying to be argumentative. Just trying to discuss "video game genres are goofy at times", since classification of them can be interesting to me.
When it comes to Half-Life, something that immediately makes it distinctive is its narrative. It was a game that very famously introduced seamlessness to its scripted events.
If you notice, however, what makes it different than, say, a very definite shoot-em-up is how its action factors into things. When you play Half-Life, are you interested in where it goes? Are you interested in the thrill of survival? That "get out" sensation that occurs early on? In that sense, it contains elements of a thriller.
Its action elements, the shooting and crowbar-whipping, tend to be very self-defensive as well, compared to a game where the player's motivation is driven by a desire to shoot the fuck out of the opposing enemy. This is where the adventure classification comes in for me, because there's more of an overall package to Half-Life and your motivations of play aren't completely based around the idea of shooting more stuff. Although, its latter end is most definitely much more action-driven, basically the most FPS part of the entire affair, since it's very much shooting.
At the same time, one can defend FPS defining games like DOOM by spinning its atmosphere and maze-like design into a more thrilling game beyond "look and shoot" too. Although I'd still think the main draw of DOOM is still in its simple, almost arcade-style Shoot-em-Up gameplay.
There's just a lot of ways the FPS label can be spun, I suppose.