4e taught me that I don't have to have a rules degree to run a good game. I don't have to completely wing it, either. There are clear guides, transparent math, and balanced leeway that support me however I approach a session.
DMing does not have to involve an overwhelming amount of prep if you don't want it to. It's entirely up to you how much or little you want to invest in the game- and it remains successful and fun in both cases.
It also taught me that I can bend the rules with little to no adverse effect. The DM is free to do as he pleases, and the rules facilitate those changes, whether planned or on the spot.
Improvise and build on the actions of the PCs. Skill challenges are a baseline, but the DM can support the PCs into taking it so much farther- within the parameters of suggested rules and well without them (letting the players use powers, for instance, which equate to varied successes depending on their resource value; or better still simply challenging them with no regard to success or failures besides what you make of it). That said, ideas can hold as much, if not more, value than the skill rolls themselves, if that's how you want the game to play.
The game is always what you make it. I've never house ruled, tinkered, and improvised as much as I have in 4e. I love the way it enables me to play, and I've become really confident in my DMing and design skills- and more so my ability to make any session of 4e work.
The shared experience is more important than the rules, and creativity is crucial. It helped me realize I don't like rules assumptions and fluff ingrained in crunch shackling me. Oh, and REFLAVOR!
Blogs are my friends, old school and new school. 4e has a great collection of grassroots designers.
Online tools are my friends, especially Power2ool.
Sacred cows taste good post-slaughter.