The Dungeon Master is a strange creature indeed:
Face it. There are very few DMs out there who can pull a great session (or adventure, needless to say a campaign) without investing hours of prep time. Of course, we do it because we enjoy it, but it takes time. A lot of it.
When I was 15 years old, I had a lot of time on my hands. I remember writing adventures, plots and whole campaigns without even having a group to play with.
I was young, free and very creative. I filled binders with stuff just for fun.
Now, I'm 35, married with 2 wonderful kids. And the one thing I'm missing (other then sleep), is time.
I find myself at 9PM after the kids are in their beds trying very hard to keep my eyes open.
So when I sit down to prepare for the next session, I need a system that gets out of the way.
IMHO, 4e did just that.
I am hardly aware of the mechanics of the system when I write my session notes. Even when I'm planning a battle - I rarely dive into books to find the correct mix of monsters, terrain features or NPC stats. Most often than not, I'll just slap some monsters (eying the Encounter XP Cap mechanism), and worry about details during the session itself. Since everything I need during play is very neatly encapsulated in those wonderful stat blocks provided by Wizards, prep time is all about making things interesting.
Since most of the chrunch was transferred to the players' side in 4e, the DM can spend his time designing the story and focusing on the plot, and that's one of the most powerful features of D&D 4e.
Try to create a 10th level Evil Wizard NPC to be used in combat in 3.5e, and see how much time is invested (wasted) on that single aspect of a session. In 4e its just a matter of finding the right stat block, or modifying it on the fly.
The whole point of this 4-part post was to have a look at the various aspects that make a good system in my opinion, and see if we can have D&D Next NOW instead of a year from now.
IMHO, 4e is the most DM Friendly D&D version out there, and I sure hope that Wizards will keep that spirit in D&D Next.