Here are some topics I have noticed that receive a lot of conversations:
1) News of the day - The hot topic, the unusual topic
2) Weather - If it's different - storms, earthquakes etc.
3) Personal events - People tend to respond if you make it very personal - something they too can relate to.
4) Great quotes - If people can relate to the quote with an awesome image
5) Sports - Sports fans love discussing that play, last night's game
6) Sometimes a simple question is all that is needed
7) From Sagittarian - Ask for their personal opinion
8) From Erin - Correcting someone's grammer (having wrong grammer) - Interesting!
And of course respond! It's a two way street with conversations whether it's on your blog, through social networks or live.