To combine a bunch of ideas:
A three-man instance, tuned for after you get at least enough role-defining skills to function recognizably as a tank, healer, and dps - let's say level 24 or so. It's set up like Flame Leviathan in a way, if you talk to one NPC the instance runs in easy mode, if you talk to the other, it's hard mode with better loot - hell, make it purple (which matters on gear at that item level). In easy mode, the NPC comes along with you and gives tips on how the trinity of roles functions in an instance - it's basically a tutorial mission. At the end of the tutorial, you get an account-bound achievement which allows the hard mode NPC to be unlocked. The hard mode NPC will only be open for conversation if all three people have the achievement.
The kicker - killing the final boss on a particular toon is also the attunement for that toon entering the LFD channel proper for real five-man dungeons. So newbies get at least some introduction to what's going on before being thrown into SFK or Gnomer. And veterans get extra loot.