For the dull-minded: (chvd)
Half-Life is a gem, a gem that will NEVER be achieved by chvddy game studios (Ubisoft, Bethesda, Activision, etc).
For the sharp-minded (chad):
The fact that it still holds up to this day (both the original and its DLCs, and the sequel, including the episodes) should mean something to the games industry, especially the industry where gambling seems to be the only way that "developers" know about.
I'd say my least favorite part about Half-Life would be some of the puzzles there, like the teleport puzzle (in the Lambda Core chapter) in HL1, but, other than that, it still holds up pretty well.
My favourite part about Half-Life will always be the soundtracks; kudos to Kelly Bailey, Mike Morasky, and many other composers of Half-Life. The soundtracks (to me at least) give you a sharp boost in your survival skills in this game; it also sounds really cool and unique as it utilizes many samples and instruments from Zero-G to Sony (yes, Sony actually did sell some of the samples; some of them were used for the Half-Life series and Portal 1, to my knowledge).
My favourite soundtracks would be: Diabolical Adrenaline Guitar (HL1), LG Orbifold (HL2), What Kind of Hospital is This? (HL2: EP 1), Vortal Combat (HL2: EP 2), and Scanning Hostile Biodats (HL: Alyx).
The Half-Life series also helped shape the game industry as we know it, inspiring game studios like Activision, Irrational Games, and Ion Storm to create other belvoes game franchises like Deus Ex.
Overall, I think Half-Life is the Superman of the game industry and game development in general, with its cultural impact upon the release of the original Half-Life and so on.
God bless you, Gabe Newell, and please count to three already.