If we elect the POTUS solely by popular vote, the votes in small states become irrelevant and elections will be determined by the votes in a few large population states. Basically directly voting for the POTUS would disenfranchise "fly over" country and California, Texas, Florida and the northeastern states would determine the election. I wouldn't expect to see any presidential candidates even to show up in states like South Dakota or Wyoming during the campaign.
Direct election of the POTUS remembering the 2000 election could make the election uncertain. Imagine the endless recalls we saw in Florida being repeated in all 50 states in a close election.
“I wouldn’t expect to see any presidential candidates even to show up in states like South Dakota or Wyoming during the campaign.”
In two presidential campaigns, the only time Obama set foot in the state of TN was for a debate in Nashville. So much for the 57 states comment