Flexibility. This feature is what has allowed us to succeed as we have throughout Wrath and Cataclysm. As far as I can tell, you have to be flexible in one of three main ways to last as a raiding guild - and we just got our core one cut out from under us.
Method one - having a bench
This method is probably hands down the most common approach in serious raid guilds. Raiders are expected to sit out from time to time, so that an entire stable of players can get gear. Every role has at least one viable backup, and nothing short of the servers being down keeps the train from moving forward. While the reality may be that 20 of the 25 players make every raid, every night, this setup does allow for the occasional missed raid without killing the team.
Method two - multiple nights
This is the method we chose to run with. Every member of the group was willing (and able) to adjust the weekly raids to a night that lined up with the schedule of everyone else. We carry no bench, don't swap or sit out players, and (to be brutally honest) haven't had a full raid week in a long time. This is pushing us into the third option.
Method three - skipping nights
Unfortunately, this is where we're heading (hell, where we already are). A lot has changed since the launch of Cataclysm, and it's had a pretty significant impact on our schedule and ability to be flexible using Method Two. I got married, our mage changed carrers, and my healing partner got a girlfriend. So now we're down to (I think) maybe three days a week where everyone is even available to raid. This means we now have zero options if someone has to miss a raid night.
The hard truth
Unfortunately, we're at a crossroads now as a guild. We have to decide if we're willing to raid less than three nights in a week, if we want to recruit in additional players and become a 'normal' guild with loot rules and everything else, or if it's time to embrace the horror and accept that while it was good while it lasted, there's little hope.
Right now there are more guilds than ever recruiting. Finding quality players can be a challenge, and I don't know if anyone in the guild wants to really go on the recruiting push. The other issue is that most of us want to raid more than anything else, and going a few weeks with no raiding is going to be tough.
So we'll see what happens. I know that I'm personally keeping an eye on the options, though we will probably have a guild chat sometime in the future (i.e. this week) to iron stuff out.
I hate shopping for a new guild almost as much as I hate recruiting.