It never ceases to amaze me when I think of how an empty body shell is placed on a assembly line, rolls along recieving hundreds of different components, comprised of thousands of parts, supplied by hundreds of vendors, installed by hundreds of different people... And when it rolls out of the other side of the factory a few hours later, it is a running, working car that you can get in, turn the key, and drive.
It's a wonder that they don't have more problems than they do, but modern cars are very reliable, and 100k miles is really just sort of "middle-aged" for a automobile nowadays. Particularly, one that has been well maintained.
You might review which services that are recommended at 100k, in your owners manual, and make sure everything is up to date. Otherwise, I can't think of anything in particular that consistently becomes a problem at 100k.
And keep enjoying your Mustang!