And a lot of people never had a problem with it.
It was worse during 2003-2004. The biggest headache at that point was the software. Ford kept doing reflashes and it still ran badly. I had one customer who specializes in the 7.3 (His race car evn uses 7.3 as it's number LOL) and Ford diesels in general say to me "You guys can't get them to run but WE can!" and laughed.
Later on it was stuff like weak head studs, and gaskets and oil leaks around the bellhousing. There were even stories about them leaking on the dealer's lots.
Now, if I recall part of this right (?), Ford DID issue a repair but the truck had to be sold first as it wasn't technically covered under warranty until actually sold? Something like that.
I also watched a salesman at the dealer I work at one cold morning start a new truck up (I'm assuming for a customer to test drive) and leave it idling. Half hour later when I was loaded up and leaving I saw it was still idling. Now I don't know if you're aware of this but I also recall that during break in Ford suggests never idling more than 5 minutes!! With light truck diesels, this is a pretty general rule anyways. My thought upon leaving was "I'm sure glad I'M not buying THAT truck!!"
And yes, some of the problems with the 6.0 (and others) is "self inflicted" like EGR valves plugging. Usually again from owners believing the old myth about leaving diesels idling for long periods of time. One time, this was normal practice, but with modern diesels not a very good idea. If using a power takeoff or other business related equipment where you need the truck running, it is usually reccomended to add a fast idle circuit thingie for this very reason.
I'm no expert so if I missed anything, anyone who has the info, feel free to add it. LOL.
Also: A lot of the problems with the 6.0 were when people modded them as it was already being stressed. Navistar had very few problems as they had their own software and rated the engines much lower thereby stressing them less. That's why Ford and Navistar were in a lawsuit a while ago, because each was blaming the other for the problems with the 6.0 and both were trying to get the other to pay more of the out of control warranty expenses.