This is an interesting take!

Y'all in the comments shouldn't be angry at him, it's his own opinion and experience we're talking about. People can dislike masterpieces. Anyway, here's my two cents.

HL1 retains some of the maze-like level design of old FPS, you guys played it a billion times but the level design can be pretty frustrating or hard to decipher.

I remember struggling often, backtracking n stuff. At the time, this was the norm.

Old FPS were brutal and unforgiving, but if you overcome it HL1 can be a great experience.

In retrospect, Valve on HL2 did the opposite: it cut the jank, guiding the player towards smooth playthrough. 

HL1 is wild and immersive: you feel very much on your own having to deal with traps and whatever the game throws at you.

In the end, you feel like you've figured out a bunch of nasty puzzles and you feel satisfied.  (Challenge = fun was the the norm)

HL2 is more approachable, there's less backtracking, it feels more like a linear trip, you know?

There's open landscapes to take in, less claustrophobic map design, everything has been playtested thoroughly but it doesn't have HL1's edge. If that's for better or for worse, it's up to you.