I left my warlock's guild because I thought they typed like morons (ur 2 dum 4 us!1!!). Really petty, right? Excessively lazy typing drives me nuts, so why would I put up with it in the box of green text constantly going by my lower left screen? Leaving only made me more sure; an officer immediately attacked me, throwing out all sorts of baseless generic insults (you're bad, you're stupid, you suck...). I suppose it's a little understandable when I give a semi-BS explanation like "it wasn't a good fit" as opposed to a more honest one like "guild chat causes scar tissue to form in both my visual and language brain regions."
My paladin left for two reasons; push and pull.
The push was that we weren't really getting anywhere. No one wanted to spend the time dying for progression content, so we were never going to get past Gruul unless 2.4 nerfed Mag to somewhere easier than trivial (don't misread this as me saying Mag is trivial). I didn't like the loot system (a few straws on a camel's back, not a whole pitchfork) and the leadership seemed a bit weak. They didn't seem to take anything as seriously as I'd have liked. I'm not saying I wanted everyone to be hardcore and playing all the time, but when you do play, play well and don't leave a raid after two wipes.
The pull was a few friends leaving for other guilds (we had some similar reasons among us all) and I wanted to follow them. That didn't happen because it turns out their new guild didn't need me, but they might not be needed either, so perhaps I can drag them with me.
Now I'm in Faith. In some ways it reminds me of Word of Redemption. They have a core of 5-10 people who seem close and play together a lot, including a few officers. They know their classes. They have a ret paladin that is more than happy to feed me sanctity aura. :P
My warlock is in a more casual guild. We're working through Karazhan (I was there for their first Curator kill) and gearing up in 5-mans. They seem pretty laid-back, but not stupid, which is pretty much what I want. I wouldn't have time for two super-serious guilds at once, but I do want my warlock to be able to do some raiding. Besides, I get sad when there's no green text in my chat window. :(
Come back next time for class-GM stereotyping, hunter-bashing, and maybe something worth reading. When is next time? Soon™.