This game is perhaps most famous for its gratuitous use of the word “f*ck”. Other games have since out-done it, by there are still so many F-Bombs in this game that you'll eventually stop noticing them. Thankfully, there are other things that make this game stand out at the crowded FPS bar.
Regarding gameplay, Kingpin prevents itself from being a generic shooter a number of ways. First, it’s HUB-based. Second, there are some light adventure elements, such as NPC interaction, more exploration than your average FPS, thugs that can be hired to follow the player around, the ability to upgrade weapons and basic mission objectives.
The setting is also quite unique. It’s some sort of cross between 50s and 90s ganglands, plus some steampunk thrown in. They did a fantastic job creating this atmosphereic world; it truly feels brutal.
Oh, the game also has the best flamethrower ever.
All these features combined prevent the game from being just another shooter, so if you're in the mood for a late 90s FPS with a few twists, check it out, but there are definitely better games out there.
Oh, and a note about the difficulty; many people have said the game is too hard, and that’s true, but only at the start when the player has horrible guns and armour. Hire any muscle you can, search high and low for cash and ammo, save often, and always aim for the head and you should be able to get through the early levels easier. Once you get a gun that can actually shoot straight, the difficulty becomes much more manageable.
For an interesting read, check out this article which documents what the game was SUPPOSED to be:
http://www.rockpapershotgun.com/2008/05/03/retro-kingpin-life-of-crime/