This should hopefully prove to be an interesting thread, as I think opinions will vary quite a bit from person to person.
For me I would consider under 1500 hrs to be low. Of course the newer the machine and the closer to 1500 you get the less likely I'd be to call it low hours. eg. 2006 with 1400 hrs would be high in my opinion.
1500 hrs to around 3500 hrs would be kind of middle range, again depending on age. A 1997 with 1600 hours would be low to my thinking, but a 2004 with 3500 hrs. would be leaning to the high side for me.
Anything over 3500 hrs would be getting to the high side, but if well looked after should still be in a good servicable condition, but will require a little higher level of service and maintenance work to keep it reliable.
I guess what I'm trying to say is for me at least, there is no hard and solid rule for low to high hours and each machine should be looked at individually based on it's own merrits and shortcomings.
I'm looking forward to seeing how this thread plays out and how my thoughts on the subject compare with those with more knowledge on the subject.
I agree with everything Don said.
It really depends on what you will doing with the machine. If you just want it for using around the house and not for commercial work 3000+ hours is still a useful machine.
I think it was around 1,500-2,000 hours is when a lot of commercial users get rid of their machines. This provides them with good re-sale value and gets them a new machine before they need to do any serious work.
As with any machine, if you look after it, it will last a long time. If you don't you will have never ending troubles. Some users only service their gear when there is a problem, when that happens its too late i know such a person... He had a 763 that was serviced maybe once a year if it was lucky, the engine oil was jet black and spewing out black smoke.
I have a machine that i would guess has over 5,000+ hours on it, we think it could be over 10,000 as the hour meter read 474 hrs when it died. Its on its second engine but still quite tight and the hydrostatics and hydraulics are still in perfect working order.
The engine and pins/bushings are the only parts that seem to wear out with high hours. Everything else seems to hold up pretty well.