When the Heat were engaged they were, per the eye test, the most cohesive and intimidating defensive team in the league. Yeah, they had LeBron, but they also had a team full of guys who were helping correctly and aggressively. They were also able to play good defense without using the increasingly-common and increasingly planned for Thibs-D, which helped them keep opposing teams on edge.
That said, I would love for the Celtics to snag a lights out shooter at the 2. We have some very recent evidence of how well Rondo compliments a guy with game like that.