The appeal of HL1 to me was that it was very different from other FPS games, you actually had to solve puzzles and use your mind beyond running and gunning. I loved the story line and how expansive they made Black Mesa feel, everything seemed well explained and it was (and still is) one of my favorite games. It also introduced a large modding community that created games like Counter-Strike and Day of Defeat, both of which were quite huge at the time.
HL2 on the other hand, which came out nine fucking years ago, was awesome. Obviously, looking back at it today you probably think less of it because it was essentially the ground work for future FPS games. The story wasn't as great, I'll give you that, but look at what the modding community turned into. Garrysmod alone is an excellent example of what an open modding community can do, and that is another reason why I love Valve.
I don't doubt that the release of HL3 and Source 2 will bring great things to the gaming world, just like HL1 and HL2 did. We can't be sure what it'll be, but Valve always delivers. Even if HL3 turns out to be a total garbage, generic piece of shit shooter, I wouldn't be surprised at all if something great came out of it's engine.
tl;dr
Valve delivers and mods are cool.
