For Heat fans still wondering why (a) non-Cleveland fans are pissed that LBJ went to the Heat, or (b) why this affects his legacy:
A lot of fans of other teams were sick of LeBron already before The Decision. The media had been hyping him up since high school, and so far he had nothing to show for it. Sometimes, he stepped it up in big games. He had ludicrous talent... when he chose to. But too many times, he did something much worse than simply choke in the big games... he quit. He would quit on his coach, quit on his team, make excuses for his bad games, leave the court without shaking his opponent's hand... nothing too significant by itself, but it all built up to something much bigger, an image he was beginning to project: LeBron doesn't want it enough. LeBron's a quitter. LeBron doesn't have that competitive fire. And so on.
Besides his personality and off-court snafus (his obsession with stats, for example), a perhaps smaller contingent of NBA fans were annoyed at LeBron for basketball reasons. He'd been in the league seven years without significantly improving his free throw shooting, jump shot, or post game. He got calls like nobody's business, which annoyed everyone but Cleveland fans, but superstar calls are nothing new. And yes, he was still young and would have plenty of time to work on those things when his athleticism eventually failed him. And yes, he made a ton of very impressive shots, and yes, there have always been a ton of young players whose games rested almost entirely on their physical abilities, even moreso than James. But LeBron wasn't supposed to be like other players. He was supposed to be a unique specimen, a physical freak with a peerless work ethic who chose basketball because he wanted to become the best player ever. The unchanging nature of his game hurt that, and fed into the perception that perhaps he wasn't willing to do whatever it took to improve and win a championship.
Now, after seven years all this had gotten big enough that the media started taking notice. That's nothing new for great players who don't win rings. It happens even in sports like baseball, where a single player doesn't have nearly the impact he does in basketball, and it happened to all the legends who didn't start winning right out of the gate: Wilt Chamberlain, Moses Malone, and even Jordan all got accused of being great individual players who couldn't take the biggest step and win it all. All of them eventually won, and (except sort of in Chamberlain's case) everyone pretty much forgot about it. If LeBron had stayed in Cleveland and eventually won a few rings, which was bound to happen as the league got older, we would similarly forget about LeBron's offcourt behavior and quitness, etc.
But LeBron couldn't wait. Maybe he really didn't think he could win a ring in the next ten years with Cleveland. Maybe he couldn't handle the pressure. Maybe he just wanted to chill with his friends. Whatever the reason, when LeBron made The Decision he accomplished several things at once. He made every man, woman and child in the state of Ohio his enemy. He increased his short-term chances of winning one or more rings (probably). He made a huge ass of himself in a spectacular display of lack of self-awareness. And, most importantly for his legacy, he invited the world to think whatever they wanted to think about his reasons for leaving--and to compare him to the greats and find him lacking.
The problem for LeBron's legacy wasn't just that he left Cleveland. It wasn't that he left Cleveland by free agency and not trade. It wasn't even that he left Cleveland by free agency, without telling them, and announced it on an hour-long broadcast. It was that he went to a team where he wouldn't be the de facto number one option, a team with three superstars. He could have been the man on any number of young teams one established superstar away from contention... the Knicks, the Nets, the Bulls. Instead, he hit the easy button and quit.
LeBron is still a great player with immense physical talents. If he wins rings in Miami, people will forget about a lot of the things they said about him before. In time, they'll forget about his personality and the impact on Cleveland will be forgotten by outsiders... if he wins. But people will always remember that he had the chance to be a be a legend, and he shrunk from that comparison.
Given all that, how can anyone outside the Heat not want them to fail? It would be the ultimate justice if, after leaving Cleveland on the grounds that he thought he couldn't win there, he didn't win in Miami either. It would make his gamble--which didn't seem like a gamble at the time--backfire on him more than anyone could possibly have imagined. Instead of merely not being in the "greatest of all time" conversation, he would be in the same conversation as Vince Carter--the good players who never were able to win as a number one option. The players who really couldn't take the next step. Far better than being remembered as a great player who couldn't win by himself, he would simply fade into history, never to be remembered at all. It would be sweet justice, and perhaps it would be a cautionary tale for other franchise players who decided to jump ship to what looked like greener pastures.
tl;dr: Hakeem's 2 rings >>> LeBron with 1+ rings >> Ewing's 0 rings >>>> Lebron with 0 rings = Wince.