The most important item is number 1. You need the talent. However there are exceptions.
The Billups pistons and Thomas Pistons come to mind. The Billups pistons contained a bunch of excellent defensive players that were all outside of the top 10. Isaiah was probably the only top 10 guy on his teams.
With defense and balanced scoring, it is possible to win the title. The Celtics need either an infusion of athleticism, scoring or above all: rebounding, and they could have 1 more great shot.
The alternative is that is it really really hard to land two top 10 players (or at least get the best big man in the game in the case of Duncan or Olajuwon's teams).
I'm not ready to go to lottery land for years yet.
(after all in terms of top 10 guys.... not even 1 of these guys are coming to Boston anytime soon: James, Bryant, Wade, Howard, Gasol, Nowitzki, Durant, Williams, Paul, Anthony or Bosh.)