Below is a list of priorities to care about, make sure you got the first elements in order before you start concerning yourselves about the later points.
Once a certain criteria is met, you need to keep maintaining it.
0. Raidleader or caller
Have someone who is comfortable at calling out important mechanics. I put this on point 0. because I believe that it's not even that hugely necessary, addons do most of the work for you nowadays, but someone calling things out on voice probably still makes certain that people's attention remains in the game a bit longer. (Admittedly my hottest take, don't shoot me down on it. Mostly based on personal experience and my own performance in raids).
1. Raid-roster & Bench: Despite this being the most fundamental basis of maintaining a functional raidteam, it appears many raidguilds neglect this part.
If you want to assure consistent raidprogress, you'll need a raidroster of at least 25+ people, including a third tank or person who knows and maintains experience with tanking the different bosses, and including people who can offspec into healing.
There is no greater setback to a guild's raidprogress than:
a. having to replace a tank
b. having to cancel a raid due to not being able to bring 20 people
This also means that you need to maintain a healthy "bench". Something might always come up in the middle of a raid or broader progress which causes you to lose people.
2. Raid-performance and raid-buffs: Green parses and below really don't belong in a high progress raid-team*. If people are getting green or worse parses consistently in your raidteam, it's probably time to start replacing them. The only counter to this might be requiring all the possible buffs and utility for your raidteam. But even if your only mage is consistently underperforming you might want to keep recruitment open...
*The only excuse for having bad parses is if you're specifically assigned to priority targets or responsibilities that help actually securing a kill.
3. Personal maximizing: Don't even bother starting to preach about "everyone must maximize to the best of their ability" when you don't even have the previous two points under control yet. It doesn't matter if people don't pick the best talent-points, don't play the most optimal spec or class, if they're still struggling with half-decent performance or you can barely get a full raidteam together every evening. Of course you won't be meeting the DPS or healing checks if your performance logs are grey... but changing around some talent points or telling the devastation evoker to go augmentation won't help with that.
Do you agree, disagree? I wrote this list because I'm seeing plenty of struggling guilds (read: anyone stuck from heroic raiding to never achieving Cutting Edge in mythic) handling these points with the wrong priorities.
This is obviously the psychopathic approach, there will always be nuance that make things more difficult eg. grey parsing girlfriends to give an example, but the general idea should count.