First thing you've got to realise is that every game you listed -
System Shock series, Bioshock, Assassin's Creed, Elder Scrolls, Red Dead Redemption, both Portal games, you get the idea
...owes something to Half Life. The concept of set pieces, the concept of walking into a room and being assaulted by a group of mercs who just abseiled into the room. The idea of walking into a camp in the texas wastes and suddenly being set upon by thugs bursting out of a cabin. The idea of pretty much everything in Bioshock short of the superpowers - philosophy included. It all came from what Half Life brought.
Specifically, you need to get some perspective on Half Life if you want to have any chance of appreciating it in this day and age.
Imagine Half Life was as new to us as Crysis was in 2007, Human revolution was at the end of last year. Imagine that nothing remotely like it had come before. You would have felt like you were in a
real life scientific campus, with key cards and num pads, instead of pressure pads and large glowing consoles. With scientists walking around who responded to your presence and commented on what you were doing/had done, who walked up to you and healed you for god sake!
Imagine aliens that blew chunks - individual, seemingly deliberate chunks - out of the landscape, burrowing out of walls.
Above all, imagine perfect, crystal clear graphics, utterly attractive and the equivalent to today's HDR in terms of crisp'ness. And such style! That opening sequence alone, on the tram, must have been responsible for more soiled pants than any pornstar in the history of the internet.
It was everything new and exciting about Portal, but so much more in addition. A fantastic protagonist, an enthralling story of epic proportions, an utterly serious game, with amazing sound and visuals. And so solid, such a high level of polish!
Simply put, don't give me Bioshock and Assassin's Creed as examples of 'good' games, none of them came close to Half Life because none of them, and no game since with the exception of Deus Ex, had anywhere near the jaw-dropping impact on the genre and gaming in general.
We owe half life for killzone, we owe halflife for halo, we owe halflife for crysis, we owe halflife for doom3, we owe halflife for counter strike, we owe halflife for halife 2 and portal.
And you don't even need to change your perspective that much to see this - it's an amazingly fun game even today if you allow yourself to get into it.
ps. And the frights! Oh the shocks! Some of those headcrab set pieces were filthy-frightening! Half Life 2 dropped a lot of the horror elements - even in ravenholm - that gave half life that extra dimension.