I personally don't see the problem. Ok, you can (and will) kill enemies with one shot, but honestly, that's what would happen in real life too. It's not like someone is gonna survive a good revolver shot in the head, or a swing of a katana through his chest. Plus, if you play on very hard difficulty (which you should) you are basically a glass cannon, so you get one or two-shotted from snipers, shotgunners, and revolver-welding Valentinos quite regularly. This happens even with 2600+ armor, since armor only reduces incoming damage by 1 every 40 points.
As a side note, i'd hate to have bullet-sponges as enemies. Apart from bosses, which SHOULD be bullet-sponges. Expecially the last boss. They really need to buff him, because by the time you face him, you can basically sneeze and he'll get vaporized on the spot.
Difficulty with regular mobs on the other hand should be improved by giving them better ai, not by nerfing the AWESOME, AWESOME thing that is the Comrade's Hammer. That thing is just a joy to use. All those limbs flying everywhere... Sooo much fun.