Once they severely restricted/shut off withdrawals, they were no longer an "exchange". People were no longer investing in Bitcoins facilitated through an exchange, they were investing in the exchange allowing withdrawals and making good on the promised high Bitcoin to USD values or low USD to Bitcoin values. All the time they were telling people it was a technical problem and they would make good on transactions. Given how insolvent they were, this had probably been going on for a significant amount of time or they just never had intentions of making good. Allowing deposits to continue despite the issues they faced was unscrupulous, and I believe it was likely a way for them to try to collect capital to make good on the "top of the line" and "bottom of the barrel" exchange rates that they had promised their customers, which they simply could never fulfill.
Perfect example of the mentality of a new investor at MtGox can be found on this reddit comment: http://www.reddit.com/r/Bitcoin/comments/1yw9vj/how_i_nearly...
I'll admit that not all the facts are known, and my conclusion above is essentially hypothetical based on the information known at this time. Perhaps when if we ever get access to internal communications within MtGox, we'll know the truth. Even pleading incompetence does not mean that the operators weren't unknowingly running a Ponzi scheme.
From Google:
Ponzi Scheme: a form of fraud in which belief in the success of a nonexistent enterprise (the definition) is fostered(i.e. the mechanism) by the payment of quick returns to the first investors from money invested by later investors.
Many valid things use the mechanism of new money paying out earlier investors; that alone is meaningless and not a defining trait of Ponzi's. All insurance also does this. A ponzi is literally "a form of fraud carried out by the belief in the success of a nonexistent enterprise"; that's it.