I think Washington, Cleveland, and Detroit are a good deal better than Charlotte - between Gordon, Kemba, and Jefferson, you're talking about three pretty bad defensive players (Kemba might not be horrific but he isn't good, either).
Kemba looks like Gary Payton when you compare him to Kyrie or Jose Calderon (or Will Bynum). Like you said, he's not great, but I think he's probably a bit above average when you factor in his ability to slip screens and prevent penetration so often on the pick-and-roll. When you consider what a huge part of the modern NBA that is, I think it's pretty huge. This also probably explains why the Bobs were nearly 3 points worse per 100 possessions defensively with him off the court.
Gordon and Jefferson are flammable, of course, which is why they're available. I honestly don't mind Jeff in 1v1 post situations to be honest, since he's big and strong enough to prevent deep post positions and at least makes it difficult on 5s. Where he's absolutely terrible is defending the pick-n-roll, and that's why Zeller should be such a good pick up.
Zeller long, and his agility is off the charts for a big man. He's going to blow up pick-n-rolls the way Varejao used to for Cleveland, which should improve Charlotte's D immensely. I don't even think it'll take him a lot of time to learn that part of the game - it's instincts with him.
I also have more confidence in their offense than you seem to. Jeff is going to get his 20/10 every night, and Kemba will continue to improve - especially now that he doesn't have to be the No. 1 option. I wouldn't be surprised to see him average 20/8, and shoot pretty close to 45%. Just having those two working together should be a huge boost, even if MKG and Zeller are offensive cyphers in the half-court.
Boston, Toronto, and Atlanta will have to shake up their rosters significantly before we can declare them worse than Charlotte, IMO. For the time being, the Celtics still have a respectable NBA roster with guys like Rondo, Bradley, Bass, Wallace, Sullinger, and Green. It's not a playoff team, but a squad that can win 30 games in a watered down eastern conference. Whether they trade Rondo remains to be seen, but I see no appealing reason to do so unless they're truly bent on tanking.
Come on, they're clearly hellbent on tanking! They've replaced Garnett with either Fab Melo or Colton Iverson - you or I could play the 5 vs. them. And that's the key for the Celtics: they'll have some guys who can score, but without Garnett and Pierce, they're going to be as bad defensively as we've ever seen them. Nobody there who can protect the rim, nobody who can handle pick-n-rolls, and Bradley was exposed badly in the playoffs. Anybody who goes up against him is just going to put their biggest backcourt out there and back him down repeatedly.
I think they're meat, with or without Rondo.
Toronto might want to trade Gay, but where are you going to trade him? Nobody wants to take on his max contract because he hasn't proven to be close to a max player. Atlanta still has Al Horford, one of the most underrated big men in the league, alongside Jeff Teague (he might be a free agent, not sure), Lou Williams, and I think they might have re-signed Kyle Korver today. They won't suck, even if Smith leaves.
I don't think that keeping Rudy means Toronto are going to be particularly good, or even "not terrible." They have four guys in their lineup who need the ball, and no one who I can see that's particularly adept at getting it to them. I think they max out a 35 wins.
You may have a point about Atlanta, especially if they sign-and-trade Smith for Asik (which is looking like a good possibility).
Regardless, I think Charlotte is a playoff team. If you have a 5 who can score and four other guys who can defend the pick and roll, you've got a very, very good shot at the postseason in today's NBA. I think that's what the Bobs have now.
EDIT: Really fun debate, by the way. I live for this sort of thing.