The main things you look at on most ATVs to be wearing out is brakes and bearings. If you dont have sealed bearings they tend to last about 400 miles in the mud and if you are just riding around they can last several thousand miles. Brakes depend on whether you have drum or disc. The drum brakes I used to run would only last me one ride in the deep mud, sand, and water. I have been running with disc brakes now and after 400 miles I still have plenty left.
Besides brakes and bearings you should be good. Cant really think of anything that need routine maintenance besides changing the oil and stuff. Check the diffs every one in a while to make sure they dont have water or anything in them.