I would think that if all other preventative maintenance is taken care of and there are no major failures due to improper use or neglect, the first thing that will seriously begin to show wear is the valvetrain. The 4 valve design is like on some detroit diesels with the little beam that goes from the top of the pushrod to the valves puts lateral strain on the valves, and over time I can see this causing the need for headwork earlier than say a 7.3. How many miles till this becomes a problem, I have no clue.
I had mid 600k miles on my 6.9 IDI diesel that I bought beat to **** and continued to beat on, and the heads had never been off as far as we could tell. for sure not in the last 300k or so. I personally went through two C6 autos in that truck but the motor held rock solid.