I can honestly say im not sure either. I played it and I did enjoy it, it was a fun game but it's way overhyped. The game is linear with little room to do much else but follow the path you are given.
The A.I. felt a little wonky, but they were an improvement from half life 1, thats for sure. HL 2 was legitimately scary in some parts, and they tried alot of cool things to mix up gameplay (like that sand lion level).
What I don't understand is how people praise the story and claim Gordon to be the greatest video game character of all time. A mute scientist fighting aliens in a very basic alien vs. human story. Nothing special or particularly new. Gordon isn't even relate able because you know absolutely nothing about him and he doesn't speak.
However I was pissed when HL2 ended on a cliffhanger, it's over eager for a sequel but it's been years. So many years.