At 20, there's just not many classes that have a complete loadout. Most classes which have one are healers. Combine this with how insane healers are in general, and I think it can be extremely miserable not to be either a healer, hybrid healer, or a character with some very strong potential to kill healers.
If I were rerolling (to play solo, which is mostly what I play), I would mainly consider rogue, mage, disc, mm hunter, resto, resto, mist, and moonkin.
I've also found survival to be kinda bad at 20, though I know some people really enjoy it and think it's good. The spec definitely has some strengths.
However, you can really have fun playing whatever you'd like and do okay. The bracket has not many twinks for obvious reasons.