They didn't put any thoughts into how to monetize, what is the short term goal, which customers they would target first, how to gain traction, how to validate the idea, what was the advantage over the competition, etc etc
And when I started pointing out all these things to think about they got either panicked or angry, "why are you not excited?" "Why are you trying to destroy the idea?"
For me, it's actually really hard to do the opposite and find a true non-technical co-founder. Someone who would actually produce something, bring something to the table other than their "idea" of "Uber for puppies" or "An awesome merge of Facebook and AirBnB".