I can understand those who have said they just don't like Bruce Willis. I don't care for Willem Dafoe but many people think he's great. The only movie I've liked him in was Platoon. So I understand the various opinions being posted about Bruce Willis.
Regarding the other leading men considered for the role, I don't know that they could have carried the attitude Willis did as McClane. It goes along the lines of Harrison Ford wasn't intended to be Han Solo in Star Wars. Sometimes, the person defines the role more than the role commands a person.