Warning: Spoilers Ahead
1) Take Risks
If you are a Whedonite, then you know Joss is famous for his fearless act of killing off loveable characters. Although I'm one of the "fangirls" who wail, and rant about said character deaths on the internet, I also understand the reasons behind each death. None of them are pointless. For example, Tara died so we could get Dark Willow, Fred died so we could have Illyria, Joyce died so Buffy could learn to be more responsible etc. If you need to do something drastic to move your story forward, go ahead and do it. Granted, some fans will most probably hate you for it, but doesn't that mean they cared for your character? Which in that case, you were doing something right and they will most probably follow the story to the very end. Risks won't pay off unless you take them first. Just don't go on a huge pointless killing spree purely for the shock factor.
2) The Plot is the Engine, but the Characters Drive the Story
There are three things that Joss does exceptionally well - character development, writing plot arcs and quirky dialogue. Before being introduced to his work, I simply just watched television shows. After investing my time in shows like Buffy the Vampire Slayer and Firefly, I started to appreciate the art, the scripts, the camera angles, plot arcs etc. Joss Whedon taught me that stories are not just entertainment, they are life. They are breath. They are real. He inspired me with his storytelling and helped me see it in a whole new light. I grew as a writer because of Joss. He changed my life, and he changed me.
3) Everything has the Ability to Inspire
Joss has a way of making viewers and readers relate to even the most absurd stories and characters. We're taught valuable lessons in story contexts that wouldn't be exist in the real world e.g. in "Chosen," the series finale, I felt stronger when the new slayers all around the world were activated. It's as if I was activated too. I know it sounds cheesy, but it's the truth. This man is all about empowerment, and equality. He even introduced a gay male Slayer in the Buffy comics. I love him for that.
4) People Change
Granted, it took the likes of Angel years and years to become a better human being (sorry, I mean Vampire), but he changed eventually. Joss doesn't teach us that there are good people, and bad people. He teaches us that everyone has flaws, and a darkness deep inside them, but it's up to us to do the right thing, because if "nothing we do matters, then all that matters is what we do." Right guys?
5) Anything is Possible
I am constantly amazed by things Joss Whedon does. I mean, this dude called his actor friends over to his home, and they made a movie in 12 days, not long after he wrapped up The Avengers. He wanted to do something, so he went ahead and did it. No bullshitting around. That's the kind of attitude writers and artists should have. Just go out there and do your thing.
I'm a proud Whedonite. I believed in Joss when his shows were getting cancelled left, right and centre, and I still believe in him now. He is a genius, and a wonderful human being.
Fun Fact: The Shawarma scene at the end of The Avengers was filmed just hours before the first premiere. Joss got the cast together, and they filmed it last-minute because he thought it'd be cool.