In my 40 years of video game experience I feel Killing Floor is one of the best FPS games ever made.
The depth of the game is great. As much as the game focuses on multiplayer and teamwork and the game shines in those respects.. the solo side of the game is perfectly balanced to make it a great experience also.
As you progress in difficulty the money gets less and it becomes harder to make sure you have the weapons you need when you need them, but as you progress in level your weapons become cheaper and more powerful. The enemy is varied and as you get in higher difficulties you need to develop the tactics to take the tough ones down safely. Many tactical skills can be employed beyond defending one area and who is best suited to kill what depending on choice of class. Situational tactics like target priority and when to run and how to "kite", knowing when you come round a corner to come face to face with a crowd of enemy, just what ones you want to smack once and keep going, kill outright or just keep on keepin' on. Add to that an active modding community with extra maps, game types, mod weapons, games and modded monsters... free for download.
Not that you have to play it that serious either, many people just find a level they are happy with and just kill stuff for fun... that is one of the great things about the game ... you can play it at any level you desire, but the challenge is there if you seek it.
Things I see as a big plus.
NO PvP - you will not get "spys" or cheaters spoiling other peoples fun. There is one modded game type GunGame that is Player vs Player, but the normal game dose not.
NO set enemy spawns, in games like some of the ghost recon type games, splinter cell types is that once you figure out a way it gets boring and tedious. This game is a complete open environment and the enemy can spawn just about anyplace in relation to player position, so even a game played the same way can have different combinations of enemy at any one time.
For reference, other games I consider top notch FPS - Unreal Tournament, Quake 3 and Team Arena, BF2142 I consider Killing Floor better than them all. 2000 hours in 6 months speaks for itself.... along with 600 hrs in the free SDK making new maps. If you like FPS and don't own it, you owe it to yourself to get it. I have spent $1000's over the years on games that were little better than drink coasters... the money I spent on this game has been the best entertainment value of my life!!