Here's the most important characteristic of a good guild: They aren't and have nothing to do with the Cuban Gang.
Other than that, here are few other good characteristics of a guild in my opinion:
1) The guild makes it a priority to help its members gain experience in all aspects of the game, especially in regards to sailing.
2) Guildmembers need to be loyal to the guild itself, but not fanatical in a way that causes other pirates grief.
3) A guild should be big enough that there is always at least two pirates online playing at the same time, but not big enough to where pirates in the same guild don't even know each other by name.
There are definitely a few more good characteristics of a guild that I failed to mention, but that's only because I'm tired of typing.
P.S.- I know of guilds where the guildmembers all have similar avatar names (i.e. TrueHazzard and TrueMenace), and I always thought that was appropriate for a guild.