The change (no pun intended) to a dollar coin has been a long time coming. Some of the reasons it has failed in the past are the failure to eliminate the dollar bill at the same time and the insistence on allowing the vending machine lobby too much input in the design.
The lobbyists want a coin that will fit existing machine slots, so we keep getting a coin the size of a quarter, which only confuses the public. The vending machine folks will simply have to do a re-fit; the coin needs to be clearly larger than a quarter.
While we’re at it, we should swap the sizes of the other coins to save some costs within those units: the nickel becomes the size of a penny, the penny the size of a dime and the dime the size of a nickel.
As for the worries about consumers adopting, I submit a personal experience. In 1996, I lived in Ireland (which had a pound coin, no pound note) and after a night out, I arrived home thinking I was broke. The next morning when I picked up my pants I noticed they weighed a ton; they were full of pound coins, enough for another night out. I hadn’t spent any change all night. From that point on I learned to reach into my pocket before I reached for my wallet. Everyone else will learn that, too.