Rondo is a great player - too bad he is uncoachable.
Based on what?
Won't implement a passing offense, walks the ball up the court, takes forever to initiate a play, constantly gambles on defense, frequently eschews the obvious and effective pass in favor of a "highlight" assist attempt, etc. I think all this has been discussed (and argued over) to death here. On the other hand, he has great athletic ability, rare court vision, superb passing skill, rare rebounding skill for a PG and has become a respectable jump shooter. He has very high BBIQ, but doesn't always utilize it. Superb competitior, also.
He is not nearly the player he should be, IMHO. Oh, and by the way, he's a terrible FT shooter (but you already knew that...).
So he has pros & cons just like every other player in the league. How does any of the above make him uncoachable, though?
When I think uncoachable, I think guys who are selfish, clash with teammates, fight with coaches, have a "get mine" mentality. Generally, someone who would also be considered a "cancer".
Rondo's been in the league for 8 years now. Sure, he's butted heads with both teammates & coaches at times but in a minor sense.
In terms of what he's doing on the court? I just don't see uncoachable. This season has been a combination of coming back from injury and playing with a less than stellar supporting cast. His play has been up & down and I don't think his skill set suits being THE guy but he's one helluva complementary player.
I just have never seen "uncoachable" with the guy.
Both Rivers and Stevens are avowed proponents of a passing offense (look at how the Clips play these days). Rondo didn't play that way during the last few seasons under Doc and he has continued, when available, to pound the ball up the floor, taking 12, 14, 16 seconds to begin to initiate a play. That might have worked most of the time with semi-fossilized but still great offensive options named Pierce, Garnett or Allen, but it's death on an offensively-challenged young squad. How are young players going to improve on offense in that kind of an environment?
RR's obsession with Sportscenter-worthy highlight assists is well-known to Celts fans of the last few years. Ditto for his incessant attempts to ballhawk on defense. If he doesn't make the steal, his man is going to the rim for dunk, easy shot, or assist. Those are hallmark symptoms of someone with Uncoachability Disease.
Hard to comment about off-the-court stuff, since we, as fans, are rarely privy to what really goes on, but I think it's clear that he was AWOL in LA for his birthday day off. As a Sacramento area resident, I can fully sympathize with RR for preferring to celebrate in LA, instead (

), but he could have simply asked for, and likely received, permission to stay behind on his off-game. The fact that he didn't speaks volumes about how he sees himself in relation to his coach and teammates. I would bet that he wouldn't have dreamed of doing that if PP and KG were still around.
Finally, an uncoachable player may always be a "cancer" in your eyes, but I think that's an awfully explosive metaphor to use, even if the tumor in question is not benign...

After seeing the Heinsohn, Ainge and Simmons comments recently, I have to think that the Celts organization has already made the decision to move RR on to another team. If they do, I will follow and cheer him, because in addition to his many flaws, he's also a genius player who QB'd us to a title and has provided Celts fans with many brilliant memories. I'll never forget how he played the fourth quarter against Miami in the playoffs after a dislocated elbow - what an incredible competitor! However, I think both RR and the Celts will be better served by a trade. Imagine RR on the Rockets, or even the Lakers...