Different kinds of people join hubs. I am listing a few from my understanding.
1) Basically few of them join hubs to make online money. So ideas of making business or utilization of free time to make money is one such topic.
2) Some are passionate to write and share knowledge. People passion is of large variety so you can share any topic you love
3) People get stuck in few relationship matter or work. Sometimes they need guidelines or experience of others to handle or resolve the situation.
4) If some one wants to buy any new gadget or item they want to know different collection of it even though google and other site provide the details. People prefer to know the review of it from some one who has used it.
These are the few you can start sharing. Hope I had helped you a bit . Let me know if I can be of some help