I want to start this out with a quick note that I know many of you know what you are doing but we do have a few players that could use a brush up on some things for their class, or for your alts that can go months between playing and need relearning on how to not suck.
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Sources of class information.
http://elitistjerks.com/forums.php Can't go wrong here at all. Many of the best number crunchers in wow go here and find out what is the best method of keeping shit alive, making shit die and tanking :D
http://us.battle.net/wow/en/forum/ Main wow forums, class forums and general, lots of random info here but most of the class sections have stickies with info that can help you. Though personally I always start with elitist but I find their class posts get to long and sometimes small info gets lost in the 100+ pages and is hard as crap to find what you need.
http://rawr.codeplex.com/ Rawr is a stand alone program you can use to build sets and adjust gear. It isn't up to date with all classes and specs but many of the most popular are accurate, and thoes that are not it's a great place to build a wish list.
http://www.noxxic.com/ Naki pointed this out in the forums, it looks to be a site that helps to wrangle up the info you would find in elitist jerks in abit more of a easy to read and use way. thx Naki
http://www.curse.com/addons/wow/need-to-know If you have anything you need to track get this and use it. It takes very little to learn and it is a Gigantic help by making bars up for timers you need to track. If I have a single addon that can help you significantly increase your dps this would probalby be it. Not having to check your buff bar or the bosses debuff bar for the tiny cd or to see how long till something runs out can be huge. Track dbuffs on yourself, debuffs on the boss, cooldowns, cast times, etc... Small memory footprint so it won't cripple you system. (http://rainingchaos.com/forums/viewtopic.php?f=1&t=2013 Tutorial in our forums)
http://www.curse.com/addons/wow/reforgenator Reforginator. Great addon, not perfect but it has some really good uses. Its abit out of date as to what your best secondary stats are but the huge help from it is in it's ability to help you cap expertise and hit with little waste through reforging.
http://www.curse.com/addons/wow/atlasloot-enhanced Great addon for easy ingame finding of loot. More user friendly than the journal blizz added to the game.
http://www.curse.com/addons/wow/deadly-boss-mods Deadly Boss Mods Most use this. Bigwigs alternative.
http://www.curse.com/addons/wow/big-wigs Bigwigs. DBM alternative.
Don't be a hero. Play smart and be aware of game mechanics. Dead dps is no dps, same goes for tanks and heals. Topping recounts dps does the guild no good if you spike then die. Try to never compare yourself to other classes. Because of mechanics and some classes have it easier on some fights than others. That being said compare against others of your class, if you -30% dps of someone else playing the same class then you should probably find out what you can do to close some of that gap.
Dps and heals are 50% gear and 50% what you do with that gear it's up to you to push as much potential out of what you have. If you want easy mode do lfr, if you want to see heroics you need to be heroic first.
edit to add link to needtoknow tutorial and noxxic info