I think this game could be polarizing. Not just because of the darker tone (not unusual for shooters to be dark, but specifically themes of mass shooting and then school shooting at the end), but even purely gameplay and aesthetics-wise. I could see people finding this boring or ugly or something, but it just feels like dumb fun for me. Despite being light on lore, I do love the small fragments of visible story. Journal entries show up between levels, briefly detailing aspects of Postal Dude's delusions relating to the level you're about to play. The music really works to build a fucked up atmosphere as well.
There IS a sense of humor here as well. Postal Dude has some ""badass"" voicelines, but things really amp up in the far sillier "Special Delivery" expansion. Special Delivery almost seems like the true bridge from the angst of Postal to the wacky shit of Postal 2.
As I said previously- this game is piss easy on the default difficulty. As far as I've heard, amping up the difficulty just means that the Hostiles will have better weapons and unlimited ammo. I'll be real, that just doesn't feel that intriguing to me. I found the fun in just trying to achievement hunt. There are, however, a variety of things to do here (including a gauntlet mode), so there is enough to keep one occupied. Especially for something free!!
I am currently playing through Postal 2 as well- I'd like to come back and update when I'm finished or at least way further in. They are... VERY different games to say the least. :D