These are the three things you need to succeed in raiding and pvp. Unless you are a mortal strike warrior, then it's gear, spec, and healer. I'm really just focusing on pve here, my pvp/arena world doesn't worry me. I'm not pushing great arena ratings, but I have fun and guild groups win almost every match in battlegrounds.
Gear, you need this to do your job. No matter what your spec or skill an under geared tank is not going to cut it. You can get by with lesser gear as a healer or dps, others can compensate. An under geared tank can't keep aggro off the healers, they have to do too much healing. We picked up this pretty quick in Heroes inc.
With Burning Crusade Blizzard made new ways to improve you gear, gems being the big addition. This gives players a way to fill in stats they are missing or stack their favorite stat. This puts a bigger gap between the "noobs" and more serious players. Blizzard tunes the encounters assuming people will take advantage of this, and enchants and mods. Players who do a poor job of this are going to struggle a lot more. I made a condescending post about people and their inablilty to do gear properly a couple of months ago so I won't go over that again.
Overall, I think gear is important but a skilled properly specced player is most likely going to do what it takes outside raiding to get the gear they need and get the most out of it. You can have all the fancy gear from Karazhan, if you don't enchant and gem it you'll still do bad dps, or fail at whatever it is you're doing (that's a challenge).
Skill is, in my opinion, the most important thing for raiding. I look at this broadly, I think a skilled player's gear will be done right and so will their spec. A skilled player knows what they are doing, and why. Even if it the reason you started doing it was because you saw someone better than you doing it.
My short career as a combat rogue was a success in my opinion. I can outdps 90% of Heroes Inc even though I haven't upgraded any of his gear in 3 or 4 months (combat rogues are op, I'm not that great). I got my spec by copying the spec of top rogue in the top guild on the server and read a guide put up by a guildie on our website. My first raid, in all blues other than my s2 off hand sword and green main hand, I out dpsed people who had been gearing in Kara for months. I read a guide and did what it said, not rocket surgery. I didn't stick with the rogue long enough to get into the why.
In most cases there really is a "best" way to do things. I have never heard of anyone guessing there way to the best raid spec. The information isn't hard to find either type in you "wow and your spec" into google and you'll be there. You do have to type in your spec though, like "combat rogue" google nor any other search engine knows what spec you are. I think just about anyone who reads up on how play and does it can learn to be asset to your raid. There is a learning curve with this stuff, I know wasn't as good as I could be when I was switching between toons for raiding.
Another "skill" that is a must the paying attention and listening skill. If you need to run the wall as ranged on Gruul and it's called out on vent and never do it, please delete your account. You are hopeless (not Krawl's alt), get a first person shooter put it on god mode and face roll away. People who don't pay attention on a raid are the worst player to raid with my opinion. If you are wiping 9 or 24 other people because you are too lazy to do as the raid leader asks you need to play with others just like you, take turns wiping on easy encounters.
Spec is important, how important depends on your class. I personally wouldn't use a non raid spec in a progression raid. Some specs work for both at this point. I don't think any will in the expansion. Blizzard is putting in too many talents and making things more specialized. Shouldn't someone who specs purely for pve or pvp have significant advantage over those who don't? What's the point of speccing for arenas if some some with more raid friendly spec is just as effective as you are?
There is the issue of speccing for what you want to do. If you are speccing for pve than do it right. The make it up as you go spec has fail written all over it. Some nerd out there has figured the most effective raid spec, if you are using the character to raid use it. People love trying figure this stuff out on their own. Then form a guild called "is trying to figure out how to spec" and go raid and try a different spec every week until you figure out what someone else already knows. Please don't spec sub with daggers, it hurts my brain.
Well, I guess I ranted a bit at the end, playing with people whom you like but don't put in the same effort to succeed you do will do that to you. You really want to play with them, you really like them. Though you really hate seeing their armory and are tired of doing the same thing for 800th time so they can catch up. It doesn't matter how much you like someone if you aren't going the same direction it won't work.
I think that's been the issue with Heroes Inc, there has been a tension between the more serious and more casual players in the guild for a long time. Our most casual players have left because they can't keep up and we have had plenty of more serious players leave due to lack of progression. There will be problems until everyone is on the same page and the page the guild will be on is picked. That's all