This game represents a real milestone in the shooter world. Destructible environments and the ability to use the environment for stealth. When I first played BC2 on release day I was more excited about the future of shooters than ever before, then BF3 released and had me scratching my head, then BF4 came out and I knew my hopes were shattered. Suffice it to say I'm still playing BC2 and have uninstalled BF3 and will be uninstalling BF4 soon.
The only reason I didn't rate BC2 a 10 is because there are still some bugs that persist to this day, they are minor, but noticeable. Instead of fixing the bugs the devs decided to put time into an expansion, which still irks me to this day.
Now on to the details that make this game so amazing:
1. Destruction - pretty much complete destruction of everything on the map is possible - buildings, walls, bridges, trees. Someone camping in a building? Shoot a missile or grenade at their suspected location, chances are they will die. Hell, take down the whole building if you have to. Some people have complained that the ability to destroy everything can make for a boring map, but destroyed buildings are still useful for cover and you can even hide inside them which people don't often expect. You can even blow holes in the ground and use them for cover.
2. Stealth - besides the many builds to hide in (at your own risk, see above), you can hide in brush/bushes and trees. I can't count the times when I would be hiding in a bush and an enemy would walk right by me and I would step out and stab them in the back. This design allows the thinking player to triumph over the player that blindly pushes forward without paying attention to their surroundings. Any class can hide in this manner but if you want an extra advantage in this area, play a sniper.
3. Map design - with the exception of one map (Heavy Metal), all the maps are well designed and either have a large number of buildings or a large amount of trees/brush/bushes, or an equal number of both.
4. Well balanced vehicles - vehicles are powerful in this game, but because infantry have been given ways to evade, hide, and fight back - vehicles just don't dominate like they do in say BF3 or BF4. This means it is beneficial to use vehicles but you don't really see someone kill whoring the entire match, unless a team is being completely lazy about using anti-vehicle tactics/weapons. It is quite impressive how the devs were able to strike the perfect balance between vehicles and infantry.