I'm currently in a guild that tries to mix hard core-ness with casual-ness. That means we don't take any regard to someone's "skill" when inviting them into the guild and we don't demand of anyone to ever raid (or pvp). We don't have any demands on people in general, other than the basics, be nice.
However, we do raid. And we try to do it seriously. We have some raids with less restricitions as to skill and gear, but we do have progress raids where we try to chose the best players based on raid dynamics, skill and gear. These raids have demands, and there is some confusion as to where these demands come from.
A constant debate (argument more like) in our guild is to who "deserves" to be picked to our progress raids. What standards do we have and why? I notice that the people who aren't often picked to raids think that the raid leaders must have some personal grudge against them. This can be quite troublesome at times. But this kind of thinking is based on a common misconception and that is that everyone has the exact same requirements needed to complete a raid encounter, and any "sorting" is based solely on the personal preferences of the person who picks the raid.
I understand why people think this way, because it isn't uncommon (and it has happened to myself) that people aren't picked for something just because they're not friends enough with the raid leader. But in some guilds (like mine), and with some people, it amazes me that people honestly think that someone isn't picked for a raid mainly because the raid leader doesn't like them.
Even in really serious guilds I suppose the raid leader in fact has to pick people he doesn't like sometimes just because that player is better for the raid than some other player would've been. And this is exactly the issue. The players do not in any way decide the difficulties of a raid encounter. Blizzard has it solely on their hands which classes will end up being the best choice to complete a raid encounter. The only thing a player can change is what they end up doing with the class they have chosen, but if a class turns out to the lesser of two choices, it isn't the players fault, it is Blizzards fault.
Let's see if I can give an example.
On Sindragosa you get a debuff that will do damage to you (place a dot) whenever you cast a spell or do a skill. This debuff is placed on one melee, one dps caster and one healer every 30 seconds or so. If you're one of 3 healers there is 30% chance you'll get the debuff each time and so on. Now in our regular raid setup we usually only use one caster dps, a boomkin. The guy is a great player and has great gear, but on Sindragosa he will have the debuff 100% of the times just because he is the only one who can receive it. I don't know if this is intended or not but it happens. This means he will, despite of being a great player and having great gear, not be able to do much damage in that fight. His dps drops to below half of his normal output.
And now for the tricky part. Blizzard has put this mechanic into the fight, so there are only two things we as players can do about it - either we don't care and try to work around it, which in any case will gimp the raid total damage output. Or we (mostly he unfortunately) will bite the sour apple and take another player, someone who won't be effected by this horrible mechanic.
What we chose to do is based on one thing, and one thing only. Most people think the decision is based on whether we like this guy or not. But of course the decision will be based on whether we will complete the encounter or not. If we manage to complete even without his regular damage, then he might join as usual of course. But if we notice that there is no way for us to complete it with the current way of things... we have to either give up on the encounter forever (or till they decide to fix it) or replace him. This would have nothing to do with personal preferences. The boomkin at stake here would have -nothing- to gain from being able to join a raid which has no prospect whatsoever to complete an encounter with his presence.
This could of course happen to me, perhaps it would turn out that a priest healer just isn't good enough for a raid encounter and I have to be replaced. I have to turn my anger at Blizzard then for failing at designing raid encounters properly, not at the ones who are stuck with trying to do the best they can with the cards they're dealt by Blizzard.
If it was up to me (and anyone in my guild I am sure), everyone would be welcome to any raid encounter. Blizzard has decided that it can't be so. There is nothing we as players can do about it, except complain on the forums.
This is just an example, but it can be replicated for nearly any raid encounter. If you're turned down for something, take a moment to think about yourself. Is your gear good enough, is your experience about the encounter good enough, or your experience about raiding overall? If it isn't, what can you do to improve it? Like I mentioned here, many new players expect to get their gear and experience on expense of others who have spent countless hours gaining these. If you don't have the time to spend on learning new fights the hard way (i.e lots of wiping), then maybe you have accept that you won't be picked for the newer and harder fights at once.
And most importantly, you must come to terms with the times that Blizzard have decided that you're playing a class or spec that isn't suited for a special fight. Don't shoot the messenger.