A Ponzi Scheme is legally defined as a fraudulent investment operation that pays earlier investors returns out of the investments of subsequent investors. A promise of returns is not necessary.
Well, and how is "give us your money and you'll later get it back, just minus some fees" possibly an investment operation, be it fraudulent or not? I mean, where did MtGox ever suggest they would pay any returns, let alone where did they ever pay returns?
You bring up an interesting point. I reviewed a wiki list of Ponzi schemes and assuming that is an exhaustive list I have to agree with you that the generally accepted definition is that you must say you are doing something with the money to benefit the investors for it to be a 'Ponzi Scheme'.
The fact that it doesn't fit the wiki or generally accepted definition of a Ponzi Scheme does not rule out the possibility that it still may be considered one but after spending some more time considering it, it does not look like a true Ponzi Scheme, even if the CEO did directly steal the money and use new money to pay old BTC buyers.
Well, what pretty much rules out that MtGox will be considered a ponzi scheme is that if it were, "ponzi scheme" would essentially be a synonym for "fraud".
Suppose MtGox wasn't a bitcoin exchange, but rather sold cars, which had to be ordered and paid for in advance. Now, suppose further that the CEO "stole" cars from the company and then bought new ones using the money from new customers when he needed to deliver a car to one of the earlier customers. That's just plain old fraud, not a ponzi scheme, and it doesn't become a ponzi scheme just because it's money instead of cars - and in particular, you as an earlier customer certainly are not liable for the losses of later customers.
edit: I mean, MtGox under the assumption that the loss was not due to a technical problem, if it way due to a technical problem, it's obviously not a ponzi scheme anyway.