Personally I'm a 70 twink as you can see, thus this is the bracket I will focus on, I appreciate tips from people knowledgeable in other brackets, 80s, 19s and 24s for example. I will also include a list of the twink guilds that I know or that writes to me and asks to be included.
If you got questions after reading, add Lebro#2385 on battletag. I'll do my best to answer them.
PVE: Every class is stronger now than they were in The Burning Crusade, therefore raids are also easier than before. There is no need to gather the 25 best twinks in the world to down Kil'Jaeden or Illidan. All you need is a nice group that can hold together and follow some tactics.
When it comes to heroics, they will be easy even if you go in Brutal Gladiator gear (Buy full set with offparts for slightly more than 2k honor, I will return to this.) It is only in green gear or blue gear that they will be truly challenging. In my guild, we have a big variety of gear level when it comes to players, we have BiS (Best in Slot) geared players that do 6-7k dps or 30k hp as tank aswell as people with green gear struggling with 2k. With this mix we have downed all raids and heroics.
As raids are so much easier, we sometimes do so called challenge runs. That means that we unequip 50% of our gear and try like that, making it quite hard in some cases. Another way of making it a challenge is doing timed runs, splitting the participants in two groups and see which group is the faster. All classes are viable in pve.
The exception here is WotLK raids, (70s can enter if a 80+ enters before them.)
In these raids spell damage simply isn't viable. Regardless of how much you gem for hitting your chance will be too low. I was forced to level up a hunter in order to do icc with my guild. What you need here as dps is, as mentioned, preferably hunters as they are the best but otherwise a rogue will cut it without losing too much dps. The expertise/hit cap to aim for is 22.5% of each, gem full expertise/hit and get the trinket from Mgt hc. You can however perform decently even if you are fully agility gemmed, at least as hunter.
Stamspam is the way to go! Warrior tanks does quite a lot of dps and is the best choice because of that. No different from 70 raiding except that avoidance matters less.
Just like 70 but it's DAMN hard as far as I know.
Look up Ante Amanthes videos on youtube if you want to see some boss fights in ICC or ToC, 8/12 downed at the moment. There is a small guild named Legacy focusing purely on wotlk raids who've downed 10 bosses but I can't seem to find them on youtube.
PVP: The bracket is fairly well balanced, I would say every single class got at least one fully viable specc, in my 1500 bgs I've seen almost every single class/specc top the bg scoreboard, healers in healing and dps in dps of course. The only exception is fury warrior which is just terribly, terribly bad. At first some classes might seem terribly op but I'd say it's more about that they are very easy to play, shadow priest, prot paladin, prot warrior and rogue are the most common examples. I can tell you however that there are very few classes/speccs that have never beaten me in a duel, and none that I've never beaten.
What gear and gems should you use then? Resilience has been nerfed so stamina is the way to go for pvp healers nowadays. You should still use brutal gladiator gear as long as it got the maximal amounts of sockets possible. Spots where brutal is never BiS are the legs, cloaks and weapons from Sunwell, regardless of class. All because of more gem slots. Statswise gloves from sunwell gives 2 more sockets aswell, sometimes Brutal gloves gives a good bonus however, lower cost for an ability or so. Personally I still prefer Sunwell gear here but it depends on the class.
For dps: Go for your main damage stat, (agi, str, int) fully or mainly. Some like to go with some stamina aswell, among hunters stamina is pretty much a must in order not to get slaughtered quickly, a bit the same for warlocks. Most classes uses more damage stat gems than stamina however
For tanks (fc) Obviously stamina.