This story's from last week, but I love it anyway -- over at the Greedy Goblin, Gevlon's guild was getting a little tired of all of the achievement-checking and gear requirements for endgame raiding, and so they set out to do something that many experienced raiders might admit seems impossible: take down Yogg-Saron with nothing but blues on. That means no epics at all -- no epic gear, dropped or crafted, no epic enchants, no epic gems. They did use profession bonuses, but everyone should have access to those by now (all it takes is money, and all that takes is time). And of course, they did it: toppled Yoggy with the group you see on the page there. The combat log is also posted, and it's about what you'd expect: none of the damage numbers are crazy high, but the group works so well together and plays so evenly that they get it done. That's the message to be taken away here: gear is nice, but nothing will get you farther than a well-oiled group of solid players.
Gevlon talks more about it in this post, with a little more math that shows that while we consider gear to be a powerful indicator of a player's skill, it isn't necessarily so. Gear upgrades, even to the point of legendary items, will only update your stats to a max of around 40%, which is not insignificant, but it won't make you two or three times the player you are now. The only way to do that is by playing better: knowing your spec and how to use the bonuses you get, knowing how to deal with aggro and how to run the fights, and knowing the rest of your raid. Does your tank know who to watch and what to do when things go wrong? Does your melee know when to get out of the AoE, and does your DPS know when to drop threat and when to open up? That stuff, as Gevlon shows in a stark light, will make your raid go much farther than any epic gear ever will.