What is considered high hours depends a little bit on the motor/make/model size and how it was used. I have seen Kawasaki Mules with hour meters that read in the low thousands of hours, but their engines are designed differently than ATV motors. They are designed for long years of work and if maintained according to OM, they last for a really long time. How long they will last, I really don't kow. I only have 315 hrs on mine so far..
Oil and air filter changes are the primary aspects of maintanence on any motor, and maybe more so with atv's. ATV's are generally subject to more harsh conditions than the average pickup truck, more water, mud, dust etc.. Engine oil and filters should be regularly changed, air filters should be changed and not neglected if you want to motor to not only perform properly, but to last a good long time. Gear oil needs an occassionaly change as well as any grease fittings need to be greased and not forgotten about.
I may not have helped you at all, but didn't want to see no responses to your question.