Some things I've noticed:
More female attention by far
My goatee grayed and I got a few gray hairs on the sides of my head (ironically many women find this appealing)
I can't drink like I used to, so I stopped entirely
Better sense of direction in life
Better decision making
Stronger backbone
You fear less than you did in your 20s
The finite nature of life becomes more apparent
Problems/priorities change in scope. In your 20's it's about making enough to cover your habits, in your 30's it's about keeping a roof over your head, knowing where your meals are coming from and how your bills are getting paid, maintaining your social groups, where your career is going, etc.
Subject to judgement because I've chosen to be unmarried without children.
All in all, it's not bad so don't trip on it