Cleveland played the Celts the toughest in the playoffs, and I think they're probably the biggest threat next year.
That being said, the Lakers are basically replacing Ronny Turiaf with Andrew Bynum. That's a big improvement to their squad. (Bynum is a lot less "unproven" than you suggest; he averaged 13 / 10 last season on 63.6% shooting.)
The Lakers should be better with Bynum, and on paper, the Celtics downgraded from where they were in the playoffs. I'm hoping things blow up in the Lakers' face, but chances are that they'll be very, very good next season.