I think the series is a bit overpraised, but it was a good "played once and only once" series. To my knowledge, the series is popular for it's realistic immersion and plot. Some would also add polish too that list, but I don't see how it is any more polished then other games.
I will conside that the plot in all areas, including environments, characterization, etc, is more developed then most shooters, but it is nothing spectacular. Many will say they got upset over the twist at the end episode two, but I honestly did not feel a thing. I simply don't care for the characters that much. So the plot itself was a minor improvement over most games, but the real kick to my groin is the immersion element.
Episode 2 was the only game in the series I enjoyed enough to replay. The ant lion and strider invasion fights were awesome, and the driving-between-fights tone of the gameplay were well developed. In fact, the tunnels(except the invasion) were the only combat segments of the game I found dull. However, the replayability was largly degraded by unskippable story elements. Chapter six is a good example. It is my favorite chapter thoughout the series, yet when I want to reply the final fight, I am slowed down by ten minutes of unskippable dialouge split before and after the initial fight of the chapter. Some will call it story emphasis, but I don't think it is. Story should always be secondary to gameplay, and to be hindered by story ruins the gameplay. I don't mind having good stories, but to break gameplay is inexcuseable.
The realistic immersion is another praise I don't get. How exactly does Gordon go about his life not saying anything? How come the high trained and armed Combine can't take on a single scientist? A headshot on someone with no helment should be no problem. That last one is not really a complaint about the game, but it brings up the question as to how people can call this game the pinnacle of immersion if these small loopholes come up. I prefer games that are more cinematic, just not too heavy in it. While i'm at it, how does an interface and other gameplay elements not ruin that immersion? I do understand immersion. I can get "into" my games if I really enjoy them, but the same praise this series gets for immersion is the same thing that makes me despise its immersion. It is a broken argument. Immersion should be based on how much you enjoy a game, not restrictive gameplay elements.
I don't care for the plot. That is not a problem. The problem is the fact the games tries to force it down my throat all the time, which ruins replayability and gameplay, which in itself as quite stale. I am not going to say the gameplay in Half-Life is nothing special. Innovative gameplay does not make me prefer one game over another. What does determine my enjoyment of a game is the subcontious rush I get out of it. Call of Duty 4 can be called less innovative and even less open then other games. Yet the adrenaline rush I get is what makes me prefer it over oher titles. CoD WaW is almost exactly like CoD4 in every way, yet the feel of the game just did not match CoD4, and thus I got bored of it very quickly. This is also why Episode two was the only game in the series I really enjoyed. Something undetectable about it was better then the other games. Don't say it was the open world elements. I enjoy linear games as well as open world games, so it can't be that. Oh yah, and the gravity gun is a giant gimmick.
I honest prefer the Metroid series. I does what Half-Life can do, only better. I will compare it too the Metroid Prime games specificlly due to the first-person aspect. The MP series has better environments, mixed with a very mind blowing soundtrack that adds to the tone of game at any given time. The puzzles are better puzzles require many options. Half-Life games only have guns, which limits the complexity of puzzles. MP games also have plenty of funky gadgets, which increases puzzle quality by having more variety and complexity. The gadgets also enhance the platforming, which I hated in the original HL games. I died more from crummy platforming segments then enemies. Thankfully the source games cut down on them. The MP games also have a good story, but they are told though optional log entries, and not forced on you.
This is getting a bit ranty, if not has been, so I'l close by saying I think the HL games are nothing special to me. I highly recommend the MP games over HL games, and I hope episode three blows my mind enough to reconsider my opinion of the future of the series and Valve. As for Valve, I loved Portal and Steam is a great achievement for PC gaming.