Having the same people in the raid team for at least 2 weeks in a row makes a big difference. Knowing your teammates and how they perform really is important.
Ugh, I know what you mean. I went through 2 "raiding" guilds before settling in on my current one (actually had been in it way back when I was up-and-coming, but it wasn't serious about raiding at that point so I moved on). The first guild had some pretty decent raiders, but then the GL started to really throw his weight around through his "council", since he started being on less and less outside of raid nights. I had joined the guild at the tail-end of Wrath (thus leaving my current one), so I was in it when Cataclysm dropped. Problem was, no one really wanted to do the T11 raids, so stuff really fell apart around the time T12 (Firelands) was current. Eventually the better raiders jumped ship, but I apparently never catch wind until it's too late, but I jump to a (at the time) better raiding guild.
That one had it's own problems. I joined it partway into the Firelands patch, 4.2. I had actually brought all my toons over, but they only wanted my Paladin for her tanking. Had a fun time progressing with the guild, but eventually one of the friends I had made in that guild left, and the GL just wrote it off like it was nothing. Eventually the Co-GMs in the guild (who were good friends with the GL, and she really wasn't the hard type, so the two Co-GMs really handled most everything) started to not like my attitude, so I caught the hint and came back to my current guild, Scions. The GL (who is probably one of my oldest friends in this game) would occasionally help out the guild I was in by off-tanking or dpsing when we were short, so I was able to catch up with him and he gladly accepted me back in.
And I'm happy about our progression. :D