Friends: My congregation had anywhere between 10 and 20 kids within a year or two of my age. We could get together a game of any sort at any time. Some of those guys were the funniest people I ever knew - even 25 years later. Some were excellent athletes. We even used to go to playgrounds and challenge the worldly kids to basketball games - we'ld get out butts kicked when we went into the city.
Girls: My congregation, and some fo the surrounding halls had some of the spectacular girls. As a group, these were the best-looking girls I've ever known. A couple were the prettiest girls in their high schools - at the two proms I went to, my date was 'princess', and Vinny's (best friend) dates were a princess and a queen. These girls were at a level that wouldn't even have known my name had I not been a JW.
Independance: Believe it or not, I think they helped me to take perspective on society as a whole. Don't get me wrong, I decry the excessive separatist attitude. But I don't feel as compelled to 'go along' or agree just because that's what society does.
Skeptical view of authority: This isn't always a plus. But I did get enough insight to be able to help my daughter see the 'angle' when some of her friends tried to rope her into some religious thing or another. I let her go, but I told her exactly what to look for. When she saw it, she realized I was right (yup my teenage daughter thought I was right).