Even Hard can be hard if you've got an inexperienced team, or don't know the maps very well. I think their progression is good. Normal can be done with any PUG over lvl 3 with sometimes wiping at the end, or in the later waves if you're not teamplaying well. The jump to hard will be a wipe without a rounded group with people over lvl 3. Make the jump to hell on earth, suicide, etc. then you better know the map, know where to hold up at, know which doors to keep welded, where the trader spawns at, know what you can kill and what a different class should kill, know how to kite 2-3 flesh pounders, etc. Much more strategic...
I suggest reading up on scrakes, FPs, etc and which class does what and also how firebugs and demo men can screw up the sharpshooter or support guy that's taking down units, and also how they can be helpful. Same with zerkers and medics. Each class has a group role and an individual role in gameplay to be effective past Hard level. This is where Killing Floor shines. When you get into group dynamics of Firebug lighting the FP on fire, sharpshooter stunning him, zerker slicin and dicin, and medic popping heals while commando and support keep all the other trash mobs at bay. Stuff like that. Fun!