3.) Whether there's a rivalry between the original team and the new team Even if a player had decent reasons for leaving a team, defecting to a historic rival will demolish any good will the fans might feel toward the star.
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Examples: Johnny Damon's departure for the New York could reasonably be blamed on the Red Sox owners' incompetence, but no amount of corporate idiocy could let Boston fans ignore the fact that Damon left for the archrival Yankees.
And then sometimes playing for a rival team makes an already bad situation much, much worse. Brett Favre exasperated Green Bay Packers fans with his retirement fakeout in 2008. But his decision to play for the Minnesota Vikings, the regional nemesis of the team where he'd spent the vast majority of his career, was putrid icing on the cake of hate.
4.) How well the defector performs on his new team Leaving a team for a higher salary, or to play for an archrival team, is bad enough. But for a player to then go to that team and play better than he did on his original team--that's simply torturous for the fans of the first team.
Example: Babe Ruth played six seasons for the Boston Red Sox, and for a lot of that time he was used as a pitcher rather than a batter. After his move to the Yankees in 1919, Ruth stopped pitching and came into his own as a batter, setting records in slugging percentage, runs batted in, and, of course, home runs. Boston fans never forgave him for being better as a Yankee than a member of the Red Sox--and for beginning the Curse of the Bambino, which wasn't broken until 2004.
But in the end, nothing is more important than ...
5.) How much fans like the defector A star can leave for more money to join a better, rival team, play like a champ once he gets there, and still be adored by his original fans. How? If he possesses that ephemeral quality that's embodied by Bill Clinton but eludes Al Gore: likeability.
Examples: Jordan, Gretzky, and McNabb have fostered so much good will among the fans of their original teams because people like them. Simple as that.
As James prepares to go back toe Cleveland, of course, is at a disadvantage on each count: LeBron left Cleveland amid the protests of fans and ownership alike; Miami has a better record than Cleveland so far this season; though there's no special rivalry between the Heat and the Cavs, Cleveland has perpetual underconfidence, so anywhere James went would feel like a sting against the city; and his self-importantly arrogant behavior as he left Cleveland pretty much killed his likeability.
While we can't expect Cleveland to offer LeBron a Jordan-style standing ovation, we can ask one thing of Cavs fans: leave the batteries at home.