Specifically he's accused of:
1) Concealing the true scale of his retirement package, none of which had been paid
2) During the '08 crisis he had Nissan temporarily take on hedging contracts (used to blunt the impact of currency fluctuations of the yen) since his bank was demanding collateral he didn't have; then later securing a line of credit from a Saudi businessman who was paid $14 million by Nissan (ostensibly for regional distribution)
I'm no forensic accountant but that's not a lot of smoke, much less fire. The retirement package was still theoretical and it's hard to believe a man being paid $20 million a year was really engaging in financial skullduggery for what amounted to peanuts of his net worth.
As others in this thread have pointed out, this seems to be a case of the Japanese looking down on greed, whereas it's the norm in America and some parts of Europe.
Also the Japanese were possibly reluctant to see a big Japanese company (Nissan) subsumed into a large international behemoth (Ghosn was trying to formally merge Renault and Nissan at the time of his arrest).
Ghosn is probably right that he can't get a fair trial in Japan's notoriously prosecutor-friendly justice system. Even the Businessweek article notes the conviction rate is at or near 100%.
Ghosn may have been arrogant and greedy but unless new details are presented the publicly available evidence suggests these charges are extremely flimsy.