Alright. I'll go ahead and lay this out as best I can. I find it absolutely insane that because I'm capable of healing from my usual shadow spec on my priest that I've accomplished more on her in 1 year than I have on my mage I've had since WoW first was released! What this all boils down to is DPS is a dime a dozen and if you suck, there's always someone else better to replace you.
I don't mind healing for my husband and his brother and friend and our mutual friend in Heroics, but don't expect me to in a raid. Even in a raid my shadow spec is needed more than my healing spec, all because it boosts the healers in their attempt to keep everyone topped off. Plus they think my buff from my damage is smexy. So I'm gonna go ahead and explain it for all of you who are DPS but feel like you just can't get noticed no matter what you do.
Warlocks: You are pretty much DPS. The only thing with you is that you have the ability to make cookies, summon people, soulstone, and benefits from pets. This is important. If you are either affliction or destruction, I suggest you duel spec to have a demonology spec as well. For raiding purposes, pets are good. But you also need to work on your gear and stats. crits are your friend and if you need a good guide on what you should get gear wise I reffer you to a warlock I know who has his stuff straight.
Mages: You too are also just DPS, but it's how you like to play the game. Fire mages are burst damage bent on insta-kills. Ice mages are controlers who are often used as kiters or in charge of controling adds. I think of Arcane as a helper tree. It boosts your damage, resistances, stats, etc. I suggest having an ice AND fire build. Depending on what your raid is asking for and what you're good at. Also, I realize that arcane powder costs money, but for the love of cake, summon a table for your grps and BG's...not just your raids.
Rogues: You are the strict DPS melee class (aside from DK's). I've come to understand that depending on your weapon is what you offer. Maces are mainly lockers. They keep things stunned and still. Daggers are the insta-killers. Fists are the fast controling insta-killers. Swords can, in some cases, be like an OT (this depends on the situation and what's involved). I have a swords rogue and because my dodge, agility, and streangth are high, I'm able to steal agro for a few seconds to help the tank and healer. But this is a system you need a LOT of practice in so don't do it unless your group says you can try. Assassination is the insta-kill build. Stealth is the sustained DPS build. Combat is your weapon enhancements mainly.
Death Knights: I'm only gonna say this once. WATCH YOUR AGRO AND GET SMARTER PEOPLE! You have no idea how many idiotic DK's I've run into who think they're cool and can do it all. Just because you're considered a "Hero Calss" does not mean you can do it all yourself. Stop stealing agro from the tank, watch your threat, be smarter with your runes and use them properly instead of wasting it senselessly! I've whiped WAY too many times cus of you idiots (and that doesn't include the ones who KNOW their role). Blood is considered the solo build, being able to heal yourself through it's abilities and your runes you use. Frost is the tanking build, but please, learn how to use it effectively. Unholy is the Controler build with the added perks of pets and bubbles. Unholy is to DK's what frost is to Mages.
Warriors: The origional OG tank. Warriors have branched on to mostly DPS now though and it's not their fault they refuse to tank really. Ever since druids and paladins were shown to be as much effective as them, warriors have felt almost shunned in the tanking department. Fury is the ultimate DPS build. You can throw in arms for added effect as well. It's a sustained DPS build. Arms is considered burst DPS, much like a fire mage. And Prot is, well...you know. I suggest Duel speccing Tank and DPS. throw your pride aside for once and just do it to get DPS gear. Here's a good idea.
Druids: This is easy. Feral is the DPS/Tank spec. Balance is the purely DPS or "BOOMKIN" spec and Restoration is the Healer spec. Yup, druids can do it all. Sadly not all at once. I don't see too many cat druids anymore. They're either Boomkin, Bear or Tree form. They are well rounded with DPS, but there's the ever loveable saying "There's no BOOM in OOM". Mana spenderature is someties thin, but they get the job done. I suggest duel speccing DPS and/or tank/healer. Being versitile helps.
Shamans: Shamans are nasty when they go duel wield. Having totem power helps their cause. Though they're not capable of tanking, they are capable to be like a druid in DPS and healing. I don't see too many shamans around on my server anymore, but I remember a time when they were badass! DPS shamans are hardcore and can be deadly with the right stuff. They stand out among all the others.
Paladins: Paladin healers, competant ones, are hard to come by. If you don't have the gear or build, don't even attempt it. I suggest staying DPS and duel speccing Tank. Both are perfect for a paladin. Their DPS is amazing and their agro holding abilities are sometimes the envy of warriors. To stand out above the others though, you need make sure you can pump out decent damage WHILE not going over the agro range. Tanks get a bit upset when their agro gets stolen and they can't get it back easily.
Priests: As stated in an article on here. Discipline priests are single target healers. Holy are aoe healers. Figure out which is best for you or what your guild needs. Shadow spec. It's mainly a leveling spec, but I've come to find that raids are more than happy to have a shadow priest in their run. This is mainly because their vampiric embrace causes all the priests shadow damage to heal the party or raid over time. This helps the healers and takes pressure off of them. It's also good to have a shadow priest for their Misery talent. It causes all damage to be amplified on the target from anyone, not just the priest. Duel spec to either an aoe or single target heal and take shadow.
Hunters: You're all around DPS. No matter your build, you are entirely DPS. The only thing I have to say to you is STOP BEING DUMB AND STEALING AGRO! There is such a thing as a competant hunter, I've seen them! Crap I was in a guild that used a hunter as a puller in raids! Turn your pets growl off! How many times do you need to be told! Your pet is only for damage, not for tanking in a party or raid! And stop taking all the loot! Just cause you can use almost every weapon in the game doesn't mean you need to roll on it all!
Ok, so some of this was a bit of a rant fest and the rest was information. Basicaly, go read up as much as you can about your class. The more you know, the better you'll get. Just because you're level 80 doesn't mean you understand every aspect of your class. I've met plenty of people who are 80 who still have no clue how to play. Mainly because they were pampered by their friends and run through instances instead of taking time to learn how to fight. If you invite someone to the game, do what I do, explain the basics, help them a little in the newbie zone, then send them off into the world to learn!