But since I saw this and the legion of THIS responses, I got to ask why this? The enjoyment of designing an adventure to me comes from the story and personalities I create. The mechanics of a system are totally irrelevant to that. What does 4e give you that makes it fun to create an adventure for? Honest question, not trying to be snarky or a threadcrapper.
Well, for me, I was stuck in the mode that everything had to be statted out "correctly". If there's a magical effect, it should be based on the spells, right? The 3rd-level hedge wizard in town can't create magical rings, right? (And he can't just be a "hedge wizard", he's a 3rd-level wizard.) You can't just give a monster the hit points and feats you want, it has to be "legal"... right?
This is just plain stupidity on my part, thinking I
had to run games that way, but I never was able to get over it. Well, maybe I would have but eventually I stopped caring.
So along comes 4E and it says, "Hey, you're the DM, your job is to provide a challenge with as much flavour as possible. Don't worry about Fey monsters having low hit points, feat chains, or the magical prowess of hedge wizards as compared to PCs. Just use your imagination, follow these guidelines to get the level of challenge right, and kick some ass."
In other words: When I prep for 4E, I feel like my imagination is unfettered.