The single biggest problem with $1 coins, is that they are of similar size to quarter-dollar coins.

    26.50 mm (1.043 in) - $1.00
    24.26 mm (0.955 in) - $0.25

They feel almost identical, in your pocket, and the $1 coin is small enough to get stuck in many vending machines.

I am not exactly sure of the reason that the mint is so resistant to making the coins a bit bigger (they used to be).