I'd have done a much better job handling my personal relationships.
Nothing I did at age 25 had really any material impact on my career. I liked A2 and the people I worked with at Arbor but I can sum the career value up as "can occasionally write an authoritative-sounding comment on HN about ISP backbone networks or DDoS attacks".
The relationship stuff though, that stuff stuck. It is kind of a miracle my marriage survived 25-year-old me. Lots of my friendships didn't survive.