A stable relationship. Having someone you can count on in rough times and really enjoy life when they are otherwise. Getting past the awkward early years and feeling like you are someone's rock and they are yours. This is truly a gift. There are many paths on this one and it is a journey that will be rocky at times as well as smooth and easy. It's really painful when this doesn't happen for folks or it takes so long to find it.
Children and all that goes with it, which is overwhelmingly positive punctuated by moments of negativity (You did what?!) Seeing them growing into decent, solid citizens and knowing you were a big part of that.
Financial stability. Of course, it's all relative but even in these hard times I don't tend to think about rent, car payments. Every single dollar is still spoken for but I like feeling as though I am channeling toward priorities of my own design. Gives one the chance to be charitable and help those in need and that is good for the giver and the recipient.
Career stability like above. Again, it's pretty relative and falls into degrees, but many folks get on a professional arc that develops inertia that can be pretty difficult to veer from or even resist to a great degree. You get 20 or more years in an industry and think back and chuckle at all the hand-wringing and long nights thinking about "What I am I going to do for a job/career?"
Reduced impression managing, like Angie said. Pretty much the freedom to say it as you see it and let the chips fall where they will. Doesn't mean freedom to act like a jack wagon, rather no need to spend energy on "Does that person like me?" filling 'like' with whatever tends to be your personal button be it admire, appreciate, respect, love, or whatever. It's just more being who you are in your own skin, knowing that you cannot make everyone happy with you even if you wanted to and tried.
A modicum of wisdom. Some say that hindsight leads to foresight. You know what I mean. I say a modicum because I haven't done enough to call myself wise at this point. I'm working my way to a 'bit' on my way to 'some'. Like others have said, more thinking through situations and making more well-reasoned choices (despite the fact that I just made and online purchase that seems rash) and less trigger-pulling and picking up the consequences later.
Getting more of a focus on people and less on things and 'stuff'. I thought a lot and spend a lot of my energy in the early years about all the things I 'needed' in life, but as time goes by, those things to matter less and less. It's the time spent with quality people that you long for and those who have gone on before you that were close to you that you miss, not that particular car, television or [insert big boy toy].
I'll stop there as it's getting long, but those are off the top of my head.
Cheers