Greece's debt is to foreigners. In a foreign currency. Big, big, BIG difference.
Japan can erase its debt with a stroke of a pen. So could have Greece, actually, but at least someone would have complained.
I mean it's ridiculous really. 230% GDP debt - it's a complete joke. Everyone knows it's unpayable, if it had to actually be repaid, which it doesn't. If you are exposed to that, you deserve what you're going to get, which is nothing. But that's OK, because no-one is exposed, because it's basically the Japanese Govt writing a bunch of cheques to itself, and the only effect will be to devalue the yen, if anyone cares, which they don't.
Frankly Japan's debt is turning into a sort of glitch in the matrix where something is obviously wrong but no-one mentions it because to do so would be to prove that none of it is real anyway.