Its long been a contentious issue, the difference between a hardcore and a casual raiding guild, especially when some casual raiding guilds are raiding 7 nights a week while the hardcore guild raids for 3. Clearly these words really have no meaning, hardcore, casual etc are just terms we apply typically to people more hardcore than us (or we believe them to be so) or more casual than us, we might classify ourselves into a group to feel like we belong, or we might do it to not associate with the other group (hardcore raiders having the reputation of being quite standoffish and elitist, casuals being treated as noobs).
This applies to your guild as well, there are casual guilds, progression guilds, levelling guilds, raiding guilds etc, however at the heart of every guild that can manage to raid in the 25 man arena there is a core of players who drive it onwards. This core of players is often in the 10 - 15 people mark, they want to raid, enjoy raiding and want to push for new content, in fact many of these players could be taken from any guild, geared up and dropped into much higher level content and hit the ground running. However as they are in a lower tier guild they do not progress at this rate (nor is there an easy way to test this unless someone has a handy population of core raiders from a Kara guild I can steal, and a guild willing to loan me their accounts in BT / Sunwell).
My theory currently is, that outside of the top raiding guild, which I will cover later, there is a core of players in every raid capable guild of between 10 and 15 players that are truly what you would class as your raiders. These players are typically the ones who are well geared, fully enchanted, know the fights, come up with your strategy and support the raid. In low tier content they shoulder the personal responsibility (this is actually detrimental later on, but aids progression at the lower end of raiding) and make sure the raid works. In contrast you have the remainder of the raid, which will go from nearly as competent, to those that turn up on the night because you needed extra bodies to fill your raid. Commonly loot distribution systems favour the former, and in so doing harm the raid.
Yup, you read that right, pushing forward your core raiders, actually harms your progression, while they will take on the majority of the roles in a group, and through Badge and PvP gear can probably pull through victories where non would exist, gearing them up and maximising their dps adds a small amount over time, in contrast gearing up someone who is at the lower end of your gear / raiding scale will likely make a larger difference, helping them with rotations, optimal builds similarly. Its not to say that anyone not raiding is a bad player, there are many people that simply dislike raiding, however typically your non-core players will have spent less time than your core optimising for the raid. Optimising and helping your "worst" players makes you much less likely to wipe, everyone knows someone that has a tendency to die in fires, that never notices things, yet you can help them, and in so doing make your raid better by upto 4% (their individual contribution to the overall fight should be something like 1/25th).
Taking an example, lets say we have a 10 minute fight with no movement or suchlike involved (so its an easy bit of maths):
In other words if you can improve any player's dps by 20 you reduce the fight time by 12,000 damage (roughly 10s at T4, 2.5-7,5s at T6 for a single player), if you can do that for the whole raid, for 10 DPS classes, thats 120,000 damage over the fight. From what I know of the Hunters, they made between 100 and 200 damage each from the changes in their rotations, that is 60,000 to 120,000 extra damage each over the course of that fight, and that is a massive difference, and its a lot easier to do with the players who are slightly unoptimised than it is to do so on those who have their casts down to the last millisecond and their gear close to the limit for that progression level.
Sometimes it can be hard to see this, its easier to assume they simply suck, that they are incompetent but you need them to fill the raid, rather take the time, talk to them, look at their gear, talents, their play style and try to help, I know many good Hunters that were improved massively when one of ours took the time to go through shot rotations, clipping, optimal talent specs for each of their gear levels, they were all good Hunters, could trap and knew their role, but a small optimisation made a huge difference. This kind of thing might seem like its up to the raid leader or officers, but take it on yourself, if you see something wrong or you think is go talk about it, one of you might learn something.
I mentioned personal responsibility, often it seems easier to simply assign a role to someone from the core, someone you know will manage to click that cube even if they need to get their warlock through the cleave path of Magtheridon, avoid the curiously complex pattern of fires, survive the stone falling, and the earth quake threatening to undo their progress. In the short run this is more effective, after all you see progression because you restrict what the people who are less experience and comfortable do. This however harms you later on, most guilds have people they "carry" through content, then you hit Teron, Bloodboil, Archimonde, fights where you cannot reassign the role, fights where you need everyone on the ball and ready. Spending the time earlier on to make people comfortable with responsibility, with multiple roles in a fight and with paying attention to multiple tasks will speed up your progress through these fights, and make farm status actually achievable.
Also giving people responsibility is a great feeling, a lot of our non-core raiders hated the idea of constructs, fearing they would wipe the raid, mess up, be yelled at or blamed, yet once they got the constructs, once they saw how it worked and how it worked they wanted more, they were confident and went into the fight ready and willing, in short we improved the raid as a whole by making sure that the people we least wanted to get constructs were now confident enough to take them. I would also say it made them better players, they willingly took on responsibility and the role of wiping the raid if they failed, and they survived, they felt better, Teron turned from a fight they had to go to to something they want to do, they want to see him lying on the ground surrounded by a pile of constructs.
This aspect is noticeable in even relatively hardcore guilds, there is still a core, the non-core group is more competent as a whole, most of them want to raid and want to progress rather than being asked to come along simply to help. It does still exist however, the quiet person in the raid, the Healer whose healing seems out of line with the others, the Tank that drops on every fight, spend the time to know these people and help them, its often a simple matter to make them into a core member. We had a healer who could simply not achieve more than 30% of our top Paladin's healing scores, when one of the Healers looked at him he realised that he was casting, leaving a gap, then casting, or casting in reaction (and not weaving Holy Lights for the reduced casting speed in an emergency). Simply teaching him to not-reactive heal and to keep a heal in the pipeline took his healing from truly atrocious, to the highest in the raid eventually, letting him know what worked and why took a player we had to consider a liability into one of our best raiders. Never write someone off, a raid isn't just the hardcore, its not about trying to optimise one small aspect, but rather about optimising all the links in your chain, otherwise that one weak link if hit can send you spinning.
Hardcore Progression Guilds:
As I said, I would look at these, I haven't been in one, however I have known several, and I still see the same aspects inside them. Their non-core members are much more competent as a whole, they would be core members in any other guild, they don't mess up in general, however there is still a differentiator between the core and non-core players. However we can learn from these guilds, they take the time to make sure that even their worst members are prepared and ready, know the fights and don't go in without understanding what is needed. This level of support is very admirable and needed, in my mind any guild could achieve the goal of killing Illidan, they just need to make sure that when they go in, they go in prepared and with everyone confident and happy in their roles, if not the loss of morale is more likely to kill the raid than any gear or skill ever could.