Please excuse my ignorance of the new CBA and legal framework of the NBA....
How can Golden State afford 4 of the top 20 players in the NBA? How can Cleveland afford 3 of the top 30, and still pay guys like JR Smith and Tristan Thompson?
They can afford their players because they are willing to pay the luxury tax. Those teams are making a ton of money right now, so paying more money to keep them together makes good financial sense for ownership. Once they got the players together, keeping them is extremely easy (teams can use Bird Rights to resign players over the cap), especially for a team like GSW with Curry making only $12 million this year
I've read over and over how the Celtics need to be smart with future contracts (despite having a ton of young guys on cheap deals). I've read about how Horford isn't worth the money he's getting paid. I've read about how certain trade rumors are unrealistic because of salaries. I've read that the 5th leading scorer in the NBA is going to price himself out of Boston....
Most of these things have to do with either cap space, actual NBA trade rules, or how much ownership would be willing to pay. A lot of trade rumors are called unrealistic because, under the rules of the NBA they can't happen (rule of thumb, teams can bring in ~150% of the money they send out if they end up under the cap afterward, and 125% of what they send out if they end up over). Other trade ideas are thrown out because the salary commitments for players (ex: Omer Asik or Tyson Chandler) extend past this season, which would cut into the team's cap room in the upcoming season and possibly prevent signing a FA.
As for future contracts for IT and others, it all comes down to the salary cap and how much owners will pay. In theory, paying IT big money (especially if he didn't live up to the contract) could prevent us from signing stars in the future. It could also lead to ownership letting other players go because they don't want to pay the luxury tax to keep a non-contender together
Why do some believe that GS and Cleveland can continue to maintain talent for the next 5 years, but then have reservations about how the Celtics should spend money?
This has to do with how much ownership is willing o pay. It's one thing to go over the luxury tax when you have a contender, but it's another to do so for a team that you hope can make the ECF
What within the CBA allows certain teams to afford many of the league's top players, while other teams aren't afforded that same opportunity?
Every team IS afforded the same opportunity to afford top players - the issue is getting them to sign there instead of for a top team or for a nother non-top team
If you want to learn more about the CBA,
saltlover Larry Coon's CBA FAQ site is really helpful (don't try to read the entire thing in one sitting, but it's really nice for just going through the list of questions and picking picking out the ones that really interest you). He explains everything very well and in words that everyone can understand