I tried two times actually : something like two years ago and another time a few months ago. And both times I came to a conclusion that : it just feels dated
Don't get me wrong, I think it's part of the charm. Simplicity is sometimes a blessing and I do get a huge nostalgia factor to it. But there are way too many "quality of life" changes that I simply can't do without now.
-The polished textures and overall better looking artstyle
-The better "feel" you get with both weapons and your character (the ability to sprint, different voices and quotes...)
-The skills for each perk, making them more unique. And the quicker progression.
-Some weapons I can't do without anymore. Even though KF1 had some pretty good selections for each perk at the end of its lifetime.
However, I have to admit some things are still better in the first game. Namely the atmosphere, which felt way scarier for some reasons. The game was also somewhat harder, even though zeds didn't have extra capabilities. And finally : some maps should definitely be ported to KF2, due to how awesome and creative they were.
But still, bottom line : I don't get as much fun as I used to while playing. It's sometimes hard to go back to what feels like an "incomplete" version of a game. But I think I might pop it back in a few years. With or without friends.