The main problem, besides age, with Nash is that he needs the ball a lot to be effective. So does Melo. I don't see how Melo can be effective with a PG like that. I'm not saying Nash isn't a beast, he is, but Melo needs the ball in his hands to be effective too and he's the better scorer/offensive player here. I know LeBron and Wade are the same players but Wade took a major back seat to Lebron in order for that to work out. Nash is going to have to play off the ball a lot more or Melo is going to have to and I just don't see Melo being able to do that.
Also defensively Nash is going to get trashed nightly with the young guards on the east. Rondo, Williams, Wall, Irving, Jennings, both of philly's pgs, Rose when he comes back. That's a lot of games against really athletic pgs.
Nash would make almost everyone else better on the team though with pick and roll plays. You could see Lin do it when Melo was out. Pick and roll, hit the screener on the roll, if he's not open take your jumper/drive to basket, if that's not open hit the open shooter for the 3. I just don't see Melo being able to play in that type of offense where he doesn't have the ball the most.
I could be completely wrong though. I hope it works out, the east should be fun for a long time.