I longed for adulthood as a child, and I mostly think it's fantastic, but a few things caught me off-guard:
Friendship issues at university, and (only once) being bullied there. I had the idea that both would never happen again on leaving school, because we were all adults, and it was quite a shock that this isn't true at all.
Having to decide whether to believe what you hear. As a child, everything is gospel: what your parents and teachers say, what shopkeepers tell you, what you read in the paper. Then suddenly as an adult, many things are negotiable. I had to learn not to believe things like the Daily Mail.
I agree about the meal planning. I often think that eating is a chore, never mind cooking.
No longer having the childish excitement of simple things like playing in the playground. I remember those moments! As an adult I like to go down waterslides, because I hardly ever did as a child, but it just isn't the same doing it aged 37 as it was aged 7.
I envy the excitement on the faces of the children around me.
Being responsible for my own health. As a family we were very lucky with health and rarely needed hospital treatment, I never needed to visit A&E, but I've had to do a few of these things in the last few years, and going to hospital seemed quite alien to me.
Everything being so relentlessly complicated!