Other than their morals? Well, I suppose there are legal implications: that's known as "theft". Also there are the morals of every other employee... I believe

Other than their morals? Well, I suppose there are legal implications: that's known as "theft". Also there are the morals of every other employee... I believe Coinbase is set up with standard controls so no one person can steal funds without it being obvious to (and preferably requiring the cooperation of) other employees. I would ask a similar question: What's preventing Nationwide Insurance, Chase Bank, and e-Trade from shutting down operations tomorrow and walking away with everyone's money? The biggest difference between these institutions and MtGox is that these institutions all have regulators who have the power to (in extremis) take over the operations of the institution if it begins to show the same kinds of trouble that MtGox showed for quite some time before its collapse. Coinbase is not currently subject to any such regulation.