Johnson had better head movement and punch variety, his gloves were always active before punching while Watson was often stationary in a high guard, a sitting duck. Watson had bad body mechanics and only threw arm punches.
Well, to be fair, the general opinion was that he was robbed in the first fight against Eubank, and was winning the second before the unfortunate ending.
Johnson was a cut above Watson. So skilled and difficult to look good against. He's got an argument for having beat Toney and Jackson.