I am currently preparing my first Dungeons & Dragons 4th Edition adventure, after having taken a break of over 15 years from dungeon mastering. I'd love to share, but as my players can read the internet, I don't want to spoil anything. So instead of talking about spoilers and details, I'll talk about the concept of the adventure.
The specific circumstances of my group are that we are currently playing with a different rule-system, specifically Warhammer Fantasy Roleplay, the Enemy Within campaign. Thus if I really get to DM, it will not just be with a new campaign, but also with a new rule-system. Playing with a new rule-system in pen & paper has its challenges, as players need to know at least the basics of the rules to not spend more time looking up stuff than playing.
Thus I remembered an old idea that has been around as house-rules in several D&D editions: Level 0 characters. The idea is that the players do *not* have to make all the choices that usually go into rolling a level 1 character. Instead they will only need to make some very basic choices: Their race, and what 4E D&D now calls their "power source". The Player's Handbook has Arcane, Divine, and Martial as options, other rule supplements offer more, but I'll stick with the basics. Basically the player just needs to say whether he is some sort of magic user, or some sort of religious character, or somebody who knows how to fight.
The players then start playing with these level 0 characters, with generic low stats of 10 in everything (modified by race), and just one at-will power, based on their power source choice. The adventure not only is designed for that low level of power, but also will teach them the basics of the rules. There will be combat with miniatures on battle maps, skill challenges, hazards to overcome, role-playing, and whatever else I can think of to build an adventure with a representative sample of the game rules.
Besides teaching the rules, the players will also learn more about their chosen power sources. I've created at-will powers with a basic function, plus two optional extra functions which costs 1 action point to activate (and thus basically are encounter powers). By choosing which of these alternative options to use, the players can tend towards what 4E calls a "role". For example the magic using character can use his extra power to either deal more damage, or for some basic crowd control. By seeing what he enjoys more during play, he can then later decide to become a warlock or a wizard.
At the end of the adventure the characters will reach level 1, get bonus stats that get their characters to the stats of a normal level 1 character, and choose a class. At this point they basically create their level 1 character for real, with all feats, skills, and powers. But, and that is the beauty of the system, with a much better idea in their head what they want their character to be, because they already played him a bit. As an added bonus, the characters also gain a background story, how they got from being regular guys to becoming heroes and adventurers. And in my personal case the players also get to see how they like that rule-system and me as a Dungeon Master.