I tend to break guilds down into three basic styles of game-play. You have the focused, hard core, very serious raiding guild. Then you also have social, mostly casual guilds. Both of these guilds have a single goal and it's pretty easy to stay on target. The third type is the hybrid guild that has a casual attitude, but has a subset of serious raiders who are progress oriented. The hybrid style is wide and covers lots of guilds who vary in their breakdown between the two other classes. But, all three have to deal with the "you suck" problem. In a nutshell, how do you tell someone who is playing poorly that they are ... well, playing poorly?
If the guild is totally social and there's almost no real push to progress, the answer is really simple. You don't tell anyone they suck. It's not important to dish out six hundred and fifty DPS on Gruul in this kind of guild. If it's fun, it's ok. This can be really relaxing and nice if you care more about other things than getting new bosses down and optimizing your role in raiding. If you can't heal a normal five man instance as a healer, it's ok. Just try again and if you walk out of there without the boss dead, no big deal.
If the guild is a progression based hard core guild, you tell everyone they suck. Just kidding, you just tell most people that they suck. If the entire purpose of your guild is performance, it's really not that hard to tell someone that they are not doing as well as they should. The people applying to be in your guild, or those who have joined your guild, know what the guild is about and have the right expectations that they will be judged on their performance. If you suck, someone is going to tell you. Maybe someone will tell you that even if you don't.
Then we have the last type of guild. The hybrid guild. This guild probably has a large group of casual players who level alts and are not really trying to refine their class to it's optimum performance. It likely has a smaller group of people who raid frequently (though not daily) with the concept of actually making progress. Maybe there are "progression" raids and maybe there aren't. But when the two groups are in the same raid, with different ideas, the entire concept of "you suck" starts to flow in both directions. "Can't person X even play their class right?" "Stop telling me that I should bring flasks!" What makes it worse is when you try to keep the different styles apart, but you depend on the more numerous casual players to get close to twenty five people into a raid instance. A four hundred DPS warm body is better than a zero DPS empty slot, right? It's hard to reinforce the idea that people need to see both sides of the fence and keep in mind which side people are on.
Sometimes I wish that Blizzard did not remove attunement requirements, if only to help solve this problem. It's easier to look at someone and say "You don't have your key, yet" than to have to tell them that they suck. Those hard barriers can be a good thing, so that people can see the effort of raiding in a more tangible way. Even if it seems arbitrary and painful, good fences help delineate the difference between those whose heart is in it and those who are just along for the ride.