U.S. Army helicopters took a beating in Iraq and Afghanistan. Not so much from enemy action, although there was a lot of battle damage, but from operating in hostile physical environments. In both Afghanistan and Iraq (especially Iraq) there was a lot of sand, very fine sand that did terrible things to helicopter engines. Iraq also had very hot conditions and in Afghanistan helicopters had to work their engines extra hard to deal with flying in thinner mountain air. It's going to cost the army nearly two billion dollars, and take until 2006, to get 900 helicopters used in Iraq and Afghanistan back into good shape.
The Afghanistan and Iraq experience also changed how army helicopter pilots are trained. Pilots are now allowed to practice violent evasive maneuvers that were formerly banned from doing during training (because they were dangerous). Iraq showed that it was more dangerous to learn how to do these maneuvers for the first time while being shot at. Some of the more extreme evasive maneuvers can be practiced in simulators. But eventually you have to try it in an actual helicopter to attain full confidence in the newly developed skills. Pilots are also receiving more training on how to handle crash landing in water (and getting out of a helicopter that's under water.) Helicopter maintenance personnel are also being given new equipment and materials to make it easier to maintain equipment in very sandy conditions. Most pilots and support troops were debriefed after their service in Afghanistan and Iraq, and more changes to training and tactics will come about as a result of troop reports.