You realize there is no where for this discussion to go right? One of the problems that I have seen with people and any Half-Life discussion is somewhat based on when they played it. This is an issue of context.
If you played Half-Life and later Half-Life 2 on the PC when they came out, you understand that there really wasn't anything like it when they came out, the same could be said about Halo on the Xbox. Valve introduced ideas to the genre that are now commonplace. So if you get someone who is into FPS and they for instance only bought The Orange Box, by the time that came out they had seen so much of the same things in other FPS games that it didn't seem innovative to them.
Being a Valve Fanboy and someone who has the benefit of context here, Half-Life in its basic form is a Doom clone. Valve introduced a way of providing storytelling, pacing, and good level design that was not seen previously in other FPS games of the time. These elements are almost standards for the modern FPS. Also, if anyone made another FPS where you were a silent scientist caught in a lab where aliens invade they would just get flak for being called a Half-Life clone. It's just people improving a formula.
(Sorta OT Rant)
I never liked the concept of calling a game a clone anyway. There are only so many genre's and types of games to be made. These are all just games that are part of a similar genre. I hate when people say that 'game y' is just a clone of 'game x'. When does a game get to be its own thing? People say Dante's Inferno and Bayonetta are just God of War clones, except for the different story, weapons, characters, environments, pacing.... Oh, the control scheme and camera angles are the same that's right... Why don't we all just call every RTS game a Dune II, Command and Conquer, or a Civilization clone then? All you do is collect resources, create people and weapons then go kill the other guy right? Hell, let's just call every platformer a Mario clone (Mario wasn't the first either, just a popular choice) and then we can call every spaceship fighting game a Defender clone.... See this doesn't get us anywhere...
Depending on who you talk to there are only 3 - 12 different major themes for books / movies. Yet people keep reading them and seeing them. I mean really, how many Vampire movies have there been or movies about Boy meets Girl, Boy loses Girl, Boy gets Girl back? Can't we all just enjoy the games we have? I mean give innovation credit where it's due, but stop this pointless debating about what game is a clone of another...
(/Sorta OT Rant)