There is a very common perception that Rondo only shows up for the big games (TV and playoffs). While his competetiveness might give him an extra spark in those games, I just don't see the "lack of effort". You don't get voted to four All-Defensive teams in a row if you are a slacker.
Another perception is that he gambles on defense. Well yes, but that was always part of the Celtics defense in the Big 3 era: Allow guards to run into the wall of KG (plus whoever) and pick the ball from them, when they try to use their go to move. It gives up a lot of low percentage shots, which is a good thing. You may often see Rondo's opponent "lighting it up" but with very low efficiency. That's no coincidence.
Last season our defense started to get some cracks and the rotations were pretty bad, probably because of KG's 5+5+5 schedule, which might be good for preserving him, but was really bad for game rythm. And without KG on the floor the above mentioned defensive tactic, just doesn't work. So over the course of the season you would see Rondo staying in front of his man a lot more than before.
You can say I'm a Rondo apologist, but you have to look at context. Rondo has always had one of the better defensive ratings on the team. Since his rookie season he has been in the top three on the team in defensive win shares/48 min. In his rookie season he was no. 1, and 2007-2010 he was no. 3 behind KG and Perk. And the last three seasons he has been no. 2 behind KG. And that's a guy who doesn't show up for regular games?
Edit: Sorry for rambling, and not answering the question. I would say that getting his free throw percentage above 70, would probably do wonders for him.