[nadam]

"For example, a social network for pet owners. It doesn't sound obviously mistaken."

It is not obvious so much that actually there are several social networks for pet owners with apparently hunderds of thousands (registered) users. There is one even in Hungary where I live (only 10million people live in Hungary), but here are some english sites also:

http://webtrends.about.com/od/socialnetworks/tp/pet-social-n...

Yes probably this is not a huge market, but some people make some money off of this.

So, yes, it is totally not obvious what startup ideas are good or bad.

[chmike]

PG's analysis has a bias for Dropbox or Facebook like ideas which are true startups and VC's dream. Of course there is plenty of room between idea for startups and idea for minimum viable businesses.