I agree that a lot of this stuff is informal, especially in the startup world. However, as far as actual legal implications go, this counts as a contract. It would even count as a contract if it were spoken; verbal contracts hold as much legal weight as paper contracts. I think we all value the relaxed atmosphere in smaller tech companies, but it's very much worth remembering, I think, that emails like this have weight, and promises are promises even when the company is small. There's a certain level on which we have to maintain professionalism even if we're small and dynamic.