I can't comment on the food at Blue Moon, as I've never had any here... not even a pastry, so be aware that my rating doesn't reflect upon its food or its amount of vegan-friendliness.
However, the coffee is better than average, and it's an awesome place to people-watch, to read, and to bring your laptop, with wireless access and about 10,000 electrical outlets too. ;-) Or: you can usually sit almost anywhere and not have to drain your laptop battery, that is.
It's known as a GLBT place, but I've felt absolutely fine here as a straight person as well. It's simply a cool place in the Longfellow neighborhood.