I want to stress this isn't a "power ranking". I'm not basing my decisions solely on regular season play, but on potential playoff match-ups, trends, and general team data. If you want power rankings that rank teams based on how they are currently playing during the regular season, I strongly advise you to check out Arturo Galleti's work at Wages of Wins.
I believe the Chicago Bulls are the only team in the East who can beat Miami in a 7 games series, and its because of their defense. Since the 2010-2011 season (Thibs era) the Bulls have been the best defensive team in the NBA in terms of defensive efficiency. This year has been the exception so far, as Thibs has had to assimilate a brand new bench. Based on their progress however, its likely the Bulls could make a run at the number one spot again.
Anyway, its not just that the Bulls play great defense, its how they play it. They are currently forcing the most amount of long two pointers in the league (the worst shot in basketball), and are allowing the least amount of 3 point makes (one of the best shots in basketball). This is a formula for success, because unlike Miami who relies on athletic rotations, or the Clippers who rely on individual defensive talent, the Bulls have a system that isn't easily scrambled or exposed. Like we saw against their match-up with the Knicks, they can defend the paint without giving up open 3 pointers (unlike Miami), making the other team look like a mess on offense.
The Bulls can also do what no one in the East has shown the ability to do, and that's punish the Heat for going small. According to Hoopdata, the Bulls have been one of the top 10 teams in terms of offensive rebounding during these past 3 years. They also however, allow a low amount of transition points, meaning they can pound the Heat on the glass without giving up easy baskets.
The Bulls are currently ranked 23rd in terms of offensive efficiency, but as Rose returns and team chemistry continues to build, look for them to reclaim a spot in the top 10. In an Eastern conference where Boston and Indiana are struggling, the Bulls will be the only team that threatens Miami (or beats them).
Maybe I just don't like them, but I think the Knicks are a gimmick. They caught a lot of teams by surprise early in the season, and were shooting the lights out every night. Since then however, they have cooled considerably. Their defense which at one point was top 5, is now 13th. Teams have also realized that if they run the Knicks off of the 3 point line, then no one outside of Melo can do any damage in the paint. The numbers are astounding, as the Knicks are 5th in the league in 3 point percentage, but only 25th in paint shot percentage. This all translates to a team that is slowly being figured out by the rest of the league, and I don't think Amare Stoudemire is a cure for either of those ailments.
Another point I have to make on the Knicks, is that I don't trust Carmelo in the playoffs. Against a defense like the Bulls or Heat, where his teammates aren't getting good looks, and Amare is struggling, is when I think we'll see the real Carmelo. The guy who will go 12-28, have no interest in getting his teammates involved, play negligible defense, and ultimately fall in 5 or so games. This team can win games in the regular season, but will ultimately flame out in the playoffs, and depending on their match-up could make their exit in the first round.
Next we come to Thunder. The Thunder are a great team, but they simply can't beat the Spurs or Clippers in a 7 game series. Without James Harden going bananas in the WCF last year, the Thunder would have been swept by the Spurs. Harden is long gone, and while Kevin Martin has filled in nicely, he won't be able to stabilize the offense when Westbrook is falling apart.
The Thunder in general, continue to be who they were. An elite offensive team ( 2nd in 2012-1st in 2013) and non-elite defensive team (9th in 2011-11th in 2012). Unfortunately, "who they were" wasn't good enough last year, and in a much tougher Western Conference, won't be good enough this year.
It should be known that everything I said about Carmelo basically applies to Westbrook. I can tolerate some inefficient shooting from a star during a series, but not if its done with extremely high usage. I'm okay with 3-9 from the field, but not 9-27. Westbrook and Carmelo are 9-27 guys.
At last we come to the Spurs, and when I went back to do some extra research, I realized I was being a bit unfair to them. While I still don't think they can beat the Clippers, I have to admit that they are a top shelf team. Their offense has remained elite (4th in the league), but their defense is what really made me relent. They were ranked 11th last year, (which is why I said they wouldn't win squat) but are 5th in defensive efficiency this year. While I think the Clippers youth, star power, and depth will overpower the Spurs, they are at this point right there with Miami, Chicago, and Los Angeles. Apologies to the Spurs.
To recap, the Thunder and Knicks won't be getting past the conference semi-finals this year, the Bulls have the best chance to beat Miami, and the Spurs will be slugging it out with the Clippers for Western conference supremacy.