I played WoW on dial-up for a while, and during that time, it was close to impossible for me to raid anything but add-light 10-man content. So I did the only thing that seemed doable: leveled alts. A lot of them. While previously I had vowed to only level my rogue (my original main) to max level in any given expansion, I was suddenly the proud owner of six level 80 characters.
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.
Nowadays, the imbalance in skill or knowledge level is much higher, due in part to the game's age and equally to its accessibility. There are new players added to the game every day, and the players who have extensive knowledge tend to become more insular and ornery due to the amount of time they've spent in Azeroth. It's get-off-my-lawn syndrome, and it happens to the best of us. We get mad when players don't perform to the same standards we expect of ourselves, and it's almost entirely because we put blinders on and expect everyone's skill or knowledge level to be comparable to our own.
But the fact is that new players have to learn how to play from somewhere. It seems reasonable at first to say, "Well, all they have to do is go to Wowhead/EJ/WoW Insider/Something Awful/class blogs and read up about their class there," but new players just want to play. They don't want to spend hours theorycrafting and reading a novel's worth of game guides just to be able to play properly. And I don't blame them. I didn't want to, either.
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.