Not being able to pick it up at any time.

Not being able to return it at any time.

Getting charged for a full two days when only renting for a day and an hour.

Reserving the vehicle online weeks before, then getting to the rental office and finding that the requested vehicle won't be available for an hour or more until it's returned by the previous renter or sent over from another rental office.

Getting a vehicle that's damaged in some way that isn't visible, like grinding in the tranny, which only becomes obvious after having started one's journey.

The fear involved with reporting problems like the aforementioned problem with the tranny, out of fear of being blamed for causing the pre-existing problem, and then getting charged for it.