Do you mean wheel-based like ball mice? I've never had any of those wear out (although did see some wear on the wheels).
Now I'm getting anxious for the IBM L40!
Those little plastic looking rubber parts that run on the trackball that drive the mechanical-optic sensor, get worn and misshapen, causing the cursor to not move in a particular direction, but also tend to add drag to the ball. Also the metal rollers, or some use of course static bearings, tend to get a depression in them, or worn down eventually which adds even more resistance.
Interestingly enough, this is another version of the Wintrack, it only uses metal rollers:
http://www.hykw.com/tbfan/gallery/wintrack.shtmlMost older trackballs tend to combine the terrible need for the mechanical optical roller, and bearings, which just makes them relatively harder to push than a fully optical trackball. Those that don't, and run ball on steel bearings tend to have a lot of slippage. I'd be curious if the DT225, for as well as this type if it doesn't use rubber to ensure a good grip doesn't tend to slip a lot.