Gas goes bad after sitting for a long period, meaning that there's regular maintenance involved with any gas generator- and it's messy maintenance at that. Plus gas generators are only good as long as you have gas to power them, and if you've had three feet of snow it's impossible to nip down to the gas station for a refill, not to mention that at that point you're relying on the gas station receiving a regular shipment of gas, something that again might be affected by whatever made the power go out in the first place. Additionally, they're loud and smelly- if you've ever been in a neighborhood after a hurricane when everyone's running their generator it's obnoxious as all hell.

Natural gas generators could be a good option, but only in areas that have natural gas lines already which rules out a lot of rural areas where generators are probably going to be more important anyway.