It happens in everyone's career. Play this game long enough, and eventually you'll be faced with the question "Is this guild right for me". You'll weigh the pros and cons of sticking with it, and eventually, you'll reach the point where you pull the trigger and leave. How you leave is up to you though (unless it's a gkick), and depending on the guild and server, you might want to think twice about how you go about it.
First though, look at why you're leaving. Often times it's a simple case of walking away from a super casual leveling guild for a shot at end game raiding. It may be a difference of opinion on the way things are handled, or maybe you just can't stand seeing the idiots type in guild chat any more. Regardless of your reasons, make sure they're right for you, and make sure you aren't getting your feathers ruffled over the wrong thing.
Every guild I've ever been in has a few core elements. Officers, loot rules, and some sort of charter or mission statement. These three things are your guiding stars when it comes to being happy in the guild - read them, know them, talk to them. Most officers are willing to listen, and please - if you have a problem - bring it to thier attention. I don't know how many times I've been approached by the player on his last thread, yet this was the first I'd heard of an issue.
Before you start your talk though, make sure you have your facts straight, and be sure to pick your battles. Sinners and Saints is a NC-17 guild, and one question on our application is "are you easily offended" - right out there in the open. Now, this isn't No Dice - where racial slurs and homosexual references were made with every other breath - but we do have a guy who likes to post pictures of himself in foil underpants. It's also not uncommon to have our conversations go off the deep end of adult conversation.
Knowing this, if you come and complain that someone used words that offended you in guild chat or vent, our response will be "ok - what do you expect of me" - because we aren't going to do anything unless it was a direct attack against you. You have the choice of ignoring/muting on vent, or leaving the guild entirely.
Sometimes though a guild may just not be meeting your progression needs/desires. Maybe you want more, or maybe you can't handle the schedule, or maybe you're looking to change your main and your current guild isn't open to that.
Regardless of your reason for leaving, in the end it all comes down to how you do it. Do it right and you'll be welcomed back and included on friends lists for patches to come. Do it wrong, and we'll care your name in stone and remember you forever.
In all honesty, I don't care why you want to leave. I've quit my fair share of guilds and even managed to take a few down with me when I left as well. At this stage in the game though, I'm just looking for an honest heads up. Don't tell me you're working on finals while you're really looking at greener grass. If you're not getting the raid time you want, say so - and then if we can't work something out - I'll wish you the best of luck as I see you out the door.
My least favorite now is the "I love you all, come find me when you want to run a heroic, kisses and hugs FOR EVAH!!" post and gquit. I'm not going to look you up, and if I remember your name at all it will be with bile in my mouth. My first act is kicking all your alts out of the guild, and making sure you don't have any sleeper agents. Unless you happened to be someone that I've run with for years, you're probably going to be dead to me. Seriously, just post your goodbye if you heel you have to, and go.
Of course if you want to go out in style - I recommend talking with my friend Tobias....