I think it's a question of the tone with which he "didn't love."
There's a difference between:
"Another flat CSS theme for bootstrap? How many minutes did that take you?"
and
"There have been a number of flat CSS themes presented on this site, so I find it hard for any one of them to really distinguish themselves. Have you considered [the idea that OP thinks is better or more original or more useful]?
---
The reason it's less than ideal to weigh in with such disdain is that it discourages other people for whom creating something like that would be a MASSIVE undertaking.
Imagine a beginning designer who doesn't know anything about CSS or graphic design, so, for their initial project, they want to create a flat CSS theme. It's ambitious for them, and they're anxious to start and they log into HN and...oh. Wow. That person just made something better than I could ever make and got BLASTED.
What's the point of trying?
Comment #2 encourages them and points them towards a better suggestion. Comment #1 adds nothing.