To respond to OP: yes there are good guilds. Well, at least one. The one I'm in, obviously.
Jokes aside, while I have no idea about our skill level (I'm guessing average), the people are super nice. The guy who leads most often is extremely chill, average age of members is probably about 25, and membership is about half-half female/male including commanders, which IMO is a sign of a very healthy community. The only obligations you have to the guild is to represent it and to use a zerg-useful build during raids, and the rules on that aren't super strict i.e. no one's gonna say "no you have to use this exact build" as long as your build is in some way raid oriented.
It's a German/EU guild though.