The bigger keys are the passing skills of James (perhaps the best passing small forward of all time), the very good passing skills of Love ( especially half court) and the almost elite-level passing skill that Varejao has, with him possibly being the third-best passing center in the NBA behind Joakim Noah and Marc Gasol. The Cavs could start a group with three great passers (at a minimum), four great shooters and at least three great scorers (Irving, Love, James), all with the best playmaker in the game to create shots when needed. The only way to slow them down is if they choose to do that on their own. Problems could come if they run isolation actions, stationary dribbles and slow-developing post-ups where every defender knows what is coming; in short, the Cavs will still be a good offense if they play this way, but they won't be nearly as elite.
Instead, a splendid blend of some kind of initial action (i.e., pinch-post handoff, ball-screen action, pin down, etc.) will lead to ball movement until there is a big crease to attack via dribble or pass. This will constantly have a defense on its heels -- especially if the shooters take open 3s every chance they get, forcing opponents to extend their defense out, thereby leaving openings closer to the basket.
Passing up open shots (when the man with the ball is a good-to-great shooter) slows the offense down and gives the defense chances to recover. The Cavaliers can have the league's best offense if they give everyone the green light (minus Varejao) to take open shots from behind the line.
James' talent and mindset to get the best shot available for anyone plays perfectly in this kind of offense. To be sure, Blatt will be giving up some control by playing this way, but having the game's best (and maybe smartest) primary ball handler on the court makes up for it. James' overall basketball IQ factors in huge here, as he was smart when he entered the league but has now played in four straight Finals. His feel for when to push, when to shoot, whom to create for, and what can work best against any defense on the fly makes him like a coach on the court. As long as all of the players buy into quickly moving the ball from weapon to weapon -- while looking for quick attacks into open gaps or open shots from their top shooters -- both James and Blatt should have an easy time on that end of the court.
The Love effect
Love's contribution in the half-court game cannot be understated. He can be a strong post-up scorer. He is deadly at pick-and-pop action. Using him as a pinch-post weapon as a scorer or passer will be very effective as well, as will spacing him out to drag the opponent's best shot-blocker (when matched up with him) away from the rim. Last season in Minnesota, Love and Kevin Martin ran an effective two-man game on the perimeter, where Martin would pass to Love behind the line and then run toward him. If the defender followed Martin, he'd take the handoff and attack; if the perimeter defender took a deeper path toward the rim, Martin would slip in front of Love's man and screen him, giving Love the open look from 3-point range. That same action, with James instead of Martin, will be even more effective -- maybe even the best two-man action in the game.