The 60,000 American reservists in Iraq are going to be replaced with another 60,000. But there will be differences. The replacement reservists being called up are getting more warning, and more training on what to expect. Moreover, they are going with more appropriate equipment. The reservists in combat units will use fewer armored vehicles (M-1 tanks and M-2 Bradleys) and more armored (with bulletproof Kevlar panels) hummers. In most of Iraq, the armored vehicles are overkill, and maintaining these vehicles is a lot of work. The hummers are a lot easier to maintain and operate, and more comfortable as well.
There has always been some tensions between the active duty and reserve troops, and a lot of this has come out in Iraq. The reservists are older, more mature and experienced than your average active duty soldier. Many reservists already served on active duty, or are retired soldiers. Once the reserve and active duty troops work together for a while, the problems disappear. But senior commanders find that they have to deal with, and use, reserve and active duty units differently. The older reservists haven't got the stamina of the younger active duty troops, but you can depend on the reservists to bring more maturity and experience to a situation. Moreover, the "rah-rah" attitude that works so well with active duty troops, doesn't fly with most of the older reservists. To the army's credit, they have noted the problems with reservists in the last year, and are trying to fix things. However, it's well to remember that there were similar problems in Kuwait in 1991, and the Balkans during the 1990s, and permanent fixes never seemed to take.