Nice thread.
I definitely agree that time flies by, and we often don't take time to stop and appreciate the moment. When I look back on my 20's in many ways it was some of the greatest times of my life. Being care free, not working, not many responsibilities, living for the next party... good times indeed especially in my early 20's.
That's not to say my 30's suck in comparison though. There's something to be said about having some $$, advancing through a career, owning a home, getting married, etc.
Times are probably just as fun for me now, just in a different way.
The things I do miss are the people. As you move through your life a lot of friends come and go. I think that makes you appreciate your true friends even more, the ones that you've known for years and years and are always there for you. And at the same time you miss those people that you lost touch with for whatever reason.