Regardless of this, to say player A is better than player B just because his TEAM has made it further along in the playoffs in a different conference is not really a valid argument IMO. You have to also consider the context, the circumstances & the production that the players put up over their careers as well. Melo has been the primary scorer on his teams ever since he was a rookie. It took Joe Johnson 4 years before he became a starter, & even then he was never really considered elite.
JJ has averaged over 20 ppg in only 5 seasons. Melo has averaged over 20 ppg every season he's been in the NBA (10 straight years). JJ's career scoring average is 17.8 ppg vs. Melo's 24.7.
To be considered a top player in this league, you need to demonstrate consistency. Carmelo Anthony has proven himself to be an elite scorer in this league. The only thing Joe Johnson does better than Melo is distributing the basketball, & even that is not a big difference at all when you consider that playing as a SG he has the advantage of doing more orchestrating & setting up an offense than Melo does playing at the SF spot, just as Melo has the advantage of getting to more rebounds as a frontcourt player.