Quake 1 is a dark, Lovecraftian gothic horror shooter, while Quake 2 is an industrial military sci-fi shooter focused on fighting an alien-machine race called the Strogg.
I loved Quake 1 when it came out. It was perfect. I loved the story about ranger, slipgates and the different dimensions and runes. I loved the original soundtrack by Nine Inch Nails. It was simple, but it was perfect.
Quake 2 was dramatically different. All the sudden you were a marine fighting an alien race called the Strogg. It made no sense.