How much of the landing runway is mine? If I am cleared to land a couple of miles on final, may I use the entire runway to the departure end, or do I have to comply with "taxi instructions" the tower controller might give me like "turn right at taxiway foxtrot," while I am still concentrating on safely completing the landing? If he gives such a clearance, and I don't acknowledge for safety reasons, do I still have to comply? If I do acknowledge and do not comply, have I committed a clearance deviation? It's great to "help the guy behind me on final" but is this type of clearance even supposed to be given? If I slide off into the grass trying to comply with a clearance that has no relation to my landing calculations, who goofed up?
Everything in Dave's answer is entirely correct.  To add to it, AIM 4-3-20 says:


  The following procedures must be followed after landing and reaching taxi speed. Exit the runway without delay at the first available taxiway or on a taxiway as instructed by ATC.


(emphasis mine)

Controllers are not supposed to give pilots a turn off the runway until they've already reached taxi speed, but at busy airports, I've seen that rule stretched. You don't have to comply with any ATC instruction that you deem unsafe, but if you do and you slide off into the grass, it's your fault.