In my ownership, nothing mechanical.
The speedometer reads 10% low at all speeds, on the stock wheels and tires. The odometer similarly reads low.
This is my third used F150. My first was a beat-to-heck '84 with a six. My second was a low mileage, but absolutely bottom-of-the-line 1990, again with the six.
I bought my '98 with 45,000 miles on it, and now have 63,000 (though see above) on it. It's a very comfortable truck, though the ride is fairly bouncy. The previous owner used it exclusively for hauling his very large boat, and so the truck has helper springs as well as airbags in the rear, which may account for it.
Though I scoffed originally at a "super-cab", the rear space is very useful. The '98 has a single rear door, on the passenger side. I also have a full-size bed, so this thing is a bit of a monster, length-wise. But for me having a pickup with anything less than an 8-foot bed is pointless, I haul too much stuff around.
I demolished a parked Saturn with it--accidentally! Just caught the front bumper of it with my "side bar" step (whatever they're called) as I was hurredly trying to park. Tore the entire front off the car, just scratched the paint a bit on this machine!
I'm looking forward to 2007, when I can buy an '04 with about the same number of miles on it.
I like the XLT level of trim, though I would like the option of rubber mats on the floor instead of carpet. If you seriously use your truck, the carpet basically becomes a cleaning pad for the soles of your boots, not a pretty picture at the dump or in the barn.
The comment about demolishing the Saturn gave me a real belly laugh. I accidently did that once with my 85 chevy truck to a German car. Thanks... I needed that!