Even after I got back on actual broadband internet, Cataclysm's introduction of new races (especially Races That Are Worgen) gave me some more incentive to bring my number of max-level characters up to, well, its maximum level. So I finally listened to Matt Rossi and made a worgen warrior. He's awesome. And he tanks, a first for me. I've been leveling him almost exclusively through the dungeon finder, taking advantage of the instant queues for a dog what wears plate armor.
I'm still pretty new to tanking, but between new talent trees, heirlooms, and questing/dungeon gear with better stat balance, most low-level instances are a breeze. So I move fast. Sometimes a little faster than other people. The same kinds of people who attack from the front as a melee class or hit "need" on spirit weapons as a mage. And I would make snide remarks to those kinds of people.
Then I realized something. I was being kind of a jackass.
So of course there are going to be people who don't know basic concepts like "melee should always attack from behind to prevent avoidance." Cataclysm did a great job spelling out certain "gamey" mechanics for players, like what weapons or armor they should use, and encouraging them to do so with small benefits. But even Cataclysm doesn't divulge or accentuate the importance of every game mechanic. These are the kinds of things that players have to learn on their own.
... or, in almost every case, from other players. Looking back at my first few months as a rogue, I learned everything (correct or incorrect at the time) from other people, whether rogues in my guild or in pickup dungeons or in general chat. This was back when the game was still new, so everyone was in the same boat in terms of the level of available knowledge about the game.
The easiest way for a player to get vital information is, and should be, to mess up. If a mage rolls on a weapon with spirit, take a second to tell them that spirit doesn't benefit their class in any meaningful way. If a melee player is positioned badly, let them know that their moves will land more often when attacking from behind. If that combat rogue has a fast main-hand, impart that their main attack is better with a slower weapon. And do it nicely. Ask if they're new to the game or if this is an alt. Find ways to equate their experience with other classes, if any, to the concept they're unfamiliar with. If you get rebuffed, you get rebuffed. You did your best.
And besides: sometimes it just feels nice to help people.