What makes your guild successful?
It depends on your definition of successful ... what you are hoping to achieve.
As far as our guild is concerned, being successful is having a low turnover of members - people join and, for the most part, stay. Those who leave, generally leave because they're no longer in playing the game, not because they can't get what they want from the guild.
A major part of that is ensuring that those who join are a good fit .. that involves ensuring they see our website, know what we're about and complete the application process.
Encouraging honesty from both sides .. you can't expect to recruit loads of people on the back of misinformation, and then assume they'll stay once they learn the truth.
The same goes for those applying to join ... if you lie when you join, you will be found out at some point. It is not the basis for a successful relationship - in life or in-game.
In fact, friendly honestly, I believe, is the basis of most things. If you raid and you wipe .. and it was your fault .. put your hands up .. attempt to try better next time .. nothing worse than a festering silence 
So .. to summarise ... honesty. Not brutal, but with a bit of a laugh and a giggle .. and possibly the odd rolling of eyeballs 