I've only noticed a few downsides to my 30s so far. For the most part, I find 30-something to be a happy, confident, optimistic, and productive time. In many ways, I'm just where I imagined I would be at this point in my life. That is, except for the published novel(s) and recurring role(s) on appealing, quirky television shows.
Still, I don't have a lot to complain about.
Not a lot, but a little: a little about the perpetually-slowing metabolism.
Last year while I was out of my head with stress and worry about the interview process for my new job, I accidentally gained like fifteen pounds. I then had to go on a sensible diet where I tracked my calories and practiced moderation. That moderation bit was completely out of character and disconcerting. But, I was pleased with the result, and now whenever I get to a certain weight threshold, I use an app on my phone to help track the calories and practice the moderation.
Sadly, I've reached the threshold this morning, and I'm firing up the app. Ho hum. This is the 30s.