The Trinity and Soloing
The central tenet of the "holy trinity" approach to MMO's is that DPS should not be taking damage. Even if you personally are capable of pulling one of the mobs and soloing it, this is strongly discouraged as your healer will feel obliged to heal you (whether or not you need the help - if you are wrong and you die, your mob gets loose) and all of the mobs will survive longer because your group's DPS is split.
When Blizzard added solo play as an intentionally supported form of gameplay to the MMO genre, at WoW's launch in 2004, they had to change this model. When you are alone, there is no one else to take the damage for you. Thus, every character needed their own mechanism for avoiding, healing, or mitigating the attacks of their enemies. However, the game (and most others that have offered solo leveling since) transitions to the traditional model at max level, and players nigh universally note that the leveling experience does little to prepare new players for this shift.
A scenario group with a tank functions largely like a traditional trinity group. A scenario group without a tank functions like a trio of solo players cooperating. On paper at least there's no problem with individual players intentionally pulling aggro and using all of their tool to survive and conquer. (The one issue arises if a player sees my mage fighting stuff, assumes that I can't survive, and adds my mobs onto the ones they had already pulled for themselves, taking on more than they can tank.) Indeed, designing scenarios with large pulls of soloable mobs rather than single mobs that lone players can't survive seems to have been the whole point of the design.
This puts scenarios in an unusual niche in the overall context of Pandaria's endgame. It sounds like the intent is to offer a gradual transition into group content, with a finite endpoint as you get the requisite gear. The catch is that, while players are probably introduced to more group concepts than they would be solo, they are by design free to continue disregarding the holy trinity concept. This is a good thing if the solo playstyle is what you find fun, and you are now free to continue that fun for one more tier up the progression. This is a bad thing if the longterm goal - for yourself, the playerbase as a whole, or Blizzard - is to break players of bad aggro habits before they get to "real" group content.