Active late-night nightlife. This may be changing as public transport just started late-night service, but it will take a bit.
Year-round nice weather. Boston has seasons. Winter gets pretty harsh.
Inexpensive housing. Office space really isn't that bad, particularly if you're at a stage where working in a coworking facility is doable, but housing is just plain expensive.
Lower cost of living. We are lower than SF or NYC, but Boston does have a generally high cost of living compared to other cities.
Access to as many acquirers. Boston's a great place to start, but early-stage acquihires don't happen as often here as they do in the Valley. Not sure that's really a bad thing.
Less VC enthusiasm for consumer tech. Good companies can also find funding, but we have fewer VCs that are willing to make big bets on consumer tech / social companies.