Huge congratulations!
I'm so happy for you! This is such a great community! Your diligence in growing this community is an example to others. Through years of hard work, perseverance, and waiting, you're getting the recognition and respect a graduated site deserves. I thank all of you whose names have been mentioned, as well as those who haven't.
My opinions are the same as all the others, but I'll add them anyway!
You people do so many things well, it's impossible to narrow it down, but the things that stick out to me most:
Care for your community
This site is more like a family than a community, and there is a genuine caring for those who come. That's a rare thing in life in general, and especially on the internet, where people are frequently mean, or apathetic. You can't fake that type of devotion, and it extends to all, without regard for their level of experience, language or country of origin, or "quality" of questions and answers.
The SE system is much different than what most people are used to, and it's hard to learn. Rather than sending people back to a more comfortable, but lower quality, place, you all pitch in to help. Users aren't left hanging, and you give great personal attention to the posts. You also empower people by voting so they get the rep they need to do things on the site, and give them the ability to participate more and have fun, which translates into longtime users who tell their friends. You think carefully before closing questions, especially for new users, and when you have to do it, you still leave friendly and encouraging comments.
Even users from other SE sites are treated exceptionally well here, and look to you for guidance which you offer with gentleness and patience. I don't drive any more (have a disease which keeps me from having a license) and I've never even asked a question. When I look up things for my sweet hubby, they turn out to be duplicates because you're all so good at answering questions! Also, too often I forget to stop by to vote and occasionally edit. Yet, when I asked something of a high-rep user today, which I didn't really even have a right to ask, they responded with as much kindness as if I was an actual contributor! I guarantee you the same request wouldn't have been met with that elsewhere, even at my higher rep sites. I know that because I've done it!
You also use your meta for lively discussions in support of each other and the site. A post like this is just one example. Meta can be daunting, but when users check out this meta for the first time, they see the same spirit they find on the main site. It gives them courage to participate here too.
Care for other communities
You improve other sites too, obviously by asking and answering. Even at sites where you don't necessarily have a lot of interest, you still help by leaving kind comments, and voting.
I love Gardening and Landscaping, and spend most of my time there. We're an awesome community of people who care the way you do here. Questions are answered very fast, and both users and mods go out of their way to take time to help. Sadly though, we're a relatively low-voting site, but we've been greatly benefited by your presence there. I've watched the pattern change at least a little, especially voting for new users, which, again, translates to empowerment and a desire to hang around. We have quite a long way to go in that department, and you're helping us, and other sites as well.
I have to shout out to, and seriously thank, @Ducatikiller, who, under a different name, has almost singlehandedly raised the bar where votes are concerned. Check out all the tabs in the voting category of users. In a very short time, he slam-bammed himself (I made that word up, but I like it) into 2nd for the year and 5th of all time! I used to stay easily in that spot, and now he keeps me on my toes, as does @Ppoggio. Compared to their level of rep and length of time and participation, they have very little to gain, so their help is altruistic and not self-serving. It's that way with all of you, and extends to many sites! (Voting is private, but I have a feeling one of you is helping me on all my sites, and has pushed me over 3k at Gardening. I'm humbled and very grateful!!!)
Care for Stack Exchange
When you care for something, you nurture it, and that's what you people do. You obviously believe that SE is the best place on the internet, and it shows in everything you do.
I'm blessed to call you my friends, and wish you great success as a graduated site.