In regards to the endgame loss against Lowenthal—it's extremely unlikely that Morphy, such a calculating machine as he was, would've missed the obvious drawing line there—simply moving the king back and forth (and, if black moved to the other side of the board, simply moving the white king to c3 and shuffling it back and forth there). More likely he saw all of this right away, but didn't want the draw—and played an inferior move instead.
But why? Was it to see if he could catch Lowenthal in a mistake? Was it because Morphy didn't know any better? Or was it a throw-away out of kindness, to grant Lowenthal a minor win, in a match where Morphy was clearly dominating?
At this point in the match, Morphy was ahead a whopping 5-1. The match was for 100 pounds (a considerable amount of money, in those days). Morphy, affluent as his family was, had little need for the money, while Lowenthal's only source of income was his chess.
Whatever Morphy's reason for botching the ending (intentional, experimental, or blunder), after winning the match, Morphy, a gentleman as always, gifted Lowenthal with new furniture for his home, valued at 120 pounds—more than the sum of his winnings.
Which leads me to think that the botched endgame was a gift, too, for Lowenthal's peace of mind.
Sometimes the kind thing is to let your respected opponent win a game or two, especially if you know the outcome is already a foregone conclusion. :)
What do you know about the playing strength of an all-time great super-GM? That he can see all sorts of crazy subtle stuff that you can't, but you do understand it after it is too late? Me, too.
Anyway, I find it quite interesting that the Morphy detractors in this thread think that playing strength has very little bearing on the ability to judge another player's... uhhh... playing strength. If opinions of stronger players carry any weight, and for that reason you do not want to take my opinions seriously, then you can take those of the former world champions that BlunderLots quoted.
If you don't believe them, then I invite you to believe me, because I am quite possibly 3200 strength in theoretical discussions that can never be proven.
Dominance is dominance in any competitive field.
Regardless, I would like to hear more from other opinionated, but strong, players (not the politically correct players afraid to voice a strong opinion).