how did they determine what was controversial? ratio of likes/dislikes? topic? mixture? not sure I care so much about 'controversial' per say. But 'lively',  I think 'lively' encompasses that too tho 

 I suppose (likes vs dislikes)*activity.
I haven't used reddit for a long time, but controversial topics usually contain interesting insights. 

 i've never really used reddit outside of product recommendations / reviews, searched via google