First off there's the spotting mechanic. It renders it useless in hardcore mode which does few of the following things. It lets players move about with much less awareness. You don't have to worry about someone tagging you so you don't need to pay attention nearly as much as you normally do.
Next off it lets people exploit certain map points, meaning it encourages camping. Since there is no killcam good luck figuring out where that sniper killed you from and that touches on another balance issue with sniper rifle killing you in one hit with body shots, will cover that in a bit. The camping issue can be a real nightmare with maps that have many houses, windows and whatnot.
When you play normal mode you have to watch your minimap a LOT if you want to succeed. It makes you think more on your feet and it forces you to understand the flow of the battle.
Then you have motion mines. This mode makes them useless. It throws a big tactical element straight out of the game.
The biggest problem however is the lowered health. This is what kills most of the balance in the game. Most guns you can just spray a bit and your target bites the dust. It takes much more skill when you have to aim and when head shots make more of a difference. In Hardcore it really doesn't matter that much. You just spray a bit and your target falls. Explosives become another exploit that is harder to deal with. Take people camping a house shooting 40mm tubes refilling themselves. This kind of crap doesn't work that well in normal mode for reasons I covered. Now we have the snipers and shotguns. Those things kill in one hit to the body at all ranges with magnum ammo with one exception which I will get to after this. In BC2 I am good with a bolt rifle. There is nothing I can't kill with it if it is possible to do so. Running targets at long range, helicopter pilots etc. When you are forced to aim for the head as I have to it makes the whole usage of a bolt sniper much more difficult, skill oriented. While I can tag a running target in the body almost without effort, it just makes it too easy.
Shotguns are again a great problem with magnum ammo they kill at all ranges with body shot. (pump shotguns with slugs). When I play normal anything but CQC I am forced to aim for the head making it much harder to exploit this setup.
To counter this just run around with medic and use body armor. Enemies can't kill you in one hit (unless headshot) and you get to heal yourself. With no spotting or killcam as that kind of medic you are very hard to stop. Plus LMGs being basically an AR with a bigass magazine.