Back when Half-life was announced and shown through publications like PC gamer etc it was running on the QUAKE engine and the idea was to have something deeper built on top, ALMOST a mod/mission pack but a lot more expansive - a whole game- the QUAKE engine was designed for this very thing. Some of the first mods had them increasing texture color depth and other things but they ended up customizing it so far the HL1 engine is - while still based on QUAKE, its own beast... Here are some things that made it special at the time : The world where you walked around and NPCs would engage you , - not unheard of but not something common to quality shooters at the time-. . .Dynamic loading- the levels loaded at intervals instead of taking you out of the action. The game focused on INTERACTING with objects like pressing buttons/ using health stations and picking up and moving things. The setting was cool- the story was cool and it was all presented in what was- for its time, a top tier graphical presentation. Because it got so much RIGHT and so little wrong- it had a big following. The second game essentially was the game which spearheaded STEAM and started the PC game digital marketplace as we know it now-
Halflife and Halflife 2 were the tech beds for some of the most POPULAR and ambitious mods... Team Fortress was originally a QUAKE mod but once it moved over to the halflife /valve side of things and many many other mods that I need not name- it created a kind of "ecosystem"- The "EPISODE" games which came out finally were another foray into the world as were the PORTAL games- after which we were given nothing- cold turkey. They tried multiple times to create new halflife games but they did not live up- it would be like the final godfather sequel or like shenmue 3 or something where it goes out with a "fizzle" and you remove your good name in the process.
PLENTY Of games did "half-life" after half-life did. Sometimes games did it with better humor, plenty of times with better graphics- but they rarely hit all the buttons ... If you've played through Half-life, Opposing force, and Halflife2 and its EPISODE games- what people are excited for is a mix of the MEME of halflife-3 AND the idea that "imagine if they could wow people the way they wowed them at the time"- Someone who comes in cold with expectations and plays the Original games probably wont appreciate that those were NEW experiences (or packages of experiences) 20 -25 years ago. A quarter century ago HALF-LIFE was sharing the market with games like QUAKE 2 and GOLDENEYE and SHOGO mobile armor division (another banger by the way, but may induce cringe in some audiences)..... We were spoiled later by games which kept the good mechanics but often fell short in heart.