The above video by Propostris and Gigi, while awesome, is not safe for work.
As I've mentioned before, building a guild can be very challenging. Recruiting and retention efforts are critical. Sometimes you find members, however, that may seem to fit in, but in the end they do not. You must strike a balance between having enough guilides to get things done and a team that works well together. Your guild rules and personal interactions help determine which members are valuable members of the team and who needs a /gkick.
I have found that having a lot of guild members is generally not better than having quality guild members. There are many ways in which guild members can not fit. Sometimes folks activity times do not mesh with the guild. It doesn't really do any good to have folks tagged up but stuck PUGging raids because they can't be there for raid times. This person may not need to be removed, but don't be surprised if he or she leaves to find a guild with raid times more suited to their play times.
Skill is another common mismatch. I've encountered some people that really want to be raiders, but don't have the time or attention to do it. Someone who generally lacks raid awareness is not a good fit for a raiding guild. You don't remove someone after the first time they stand in a fire or accidentally pull a trash mob. When it becomes obvious that someone isn't interested in improving their performance, it may be time for a separation. There are some fights where a single person can wipe a raid, it's up to you to decide how many wipes are too many.
Personally, I have trouble with someone who is discourteous to the raid. Arriving late or leaving without notice frustrate the group and the guild. Chronic AFKs or constantly needing buff reminders also represent a lack of raid awareness. Folks who regularly leave raids high and dry should not be surprised when it's a while before they are asked back to raids, and if their violations are severe or frequent enough, they may be released from the guild.
Attitude is the most important thing though. Whether it's situational negativity or an over developed sense of self-importance attitude problems are toxic. There are many personalities in WoW, and the more folks in the guild the more chances there are for conflicts. But someone who is just cranky or offensive is usually just poisonous to the guild. I find that it's best to remove someone when I have multiple people complaining about a single person. No matter how good of a player it's better to remove one than alienate the many. One rotten apple can really put a damper on a guild.
It's up to you to decide how serious your guild business is. Overall, your guildies should match your expectations. My guild was formed on "people I would sit around a campfire and drink beer with." We get things done, but have a good time while we're doing it. I expect everyone to pay attention to the raid, but I am really put off when someone raises their voice in vent. Since this is what I do for fun, I enjoy a mellow environment. I do my best to keep instill that perspective in my guild and my raid.
It's not fun to remove people from your guild. I generally feel bad when I have to hit the "gkick" button, but in the end I know it's for the better. I like to believe that there's a guild out there for everyone. It's just not always mine.
WoW.com offers a plethora of information on guild leadership and guild membership. Be sure to check out Scott Andrew's Weekly Column Officer's Quarters and keep an eye on the community with Mike Schramm's Guildwatch.