Popular:
Blog Entries with a total of 10 or more Greats, Thanks, Useful, etc. - main Blog Post + Replies - are candidates for Popular. So, it may be that the original Blog Post may have 0 opinions, but 1 or more of the replies receive many positive opinions and they add up to 10 or more when totalled for all the replies. Then this entire Blog Entry will appear in the Popular tab.
.
The...
more... Hide/Show mechanism for Replies is also different for the Popular tab. Replies with 2 or more opinions are visible in the Popular tab. Other replies with no opinions are hidden by default and need to be expanded.