The aging car rental fleet
I've noticed over the past few years that my odds of getting a newer car rental have dropped greatly. In fact, it's not uncommon to get cars with 30,000 and 40,000 miles when you pick up a car rental.
With that, you get the potential problems of a heavily used car -- worn tires, aging battery, etc -- that might cause problems on the road. I've never had major problems with a car rental, but can see the potential pitfalls with this. Not sure what to do about it, but it definitely is a trend.
Wall Street Journal's Scott McCartney did a nice write up on this worth a look: http://blogs.wsj.com/middleseat/2013/08/29/rental-car-odometer-shockers/