IMO, the most likely candidates to challenge the Celtics in the playoffs this year are: Bulls, Heat. Orlando, Spurs, Mavs. and Lakers. I think the Bulls (42-18) will be the Celtic's biggest challenger in the east, with the show down coming in the Eastern Conference Finals.
Here's Hubie Brown, Mike Tirico and Bruce Bowen talking about the East. http://espn.go.com/video/clip?id=6182913
I think either the Spurs(51-11) or Mavs 45-16) will win the West. The Finals will be Celtics/Mavs or Celtics/Spurs. Either series will be epic 7 game affairs. If the season ended today, Spurs would have the best record and home court advantage. Celts (45-15) would have home court over the Mavs (but as of today, Mav's are just 1 game back in the loss column). So I'm rooting for Spurs and Mavs to lose a few games, and for whoever comes out of the West to have a lower seed (worse record) than the Celtics.
Every game is important, and one loss now could be the difference between winning and losing the title. Of course none if this matters, if the Celtics aren't healthy. To compete for the title, the Celts need Shaq and Delonte to be healthy. If that happens, only the Bulls, Mavs and Spurs stand in the way.
It's the NBA- where every game matters.