As I understand it the idea is that your email address is your identity so you only need to remember that (which most people are capable of) rather than having to a) remember your username/password (which can at least be stored in LastPass, etc for you) or b) having to remember which one of the social login buttons to click on.

If you signed up with a yahoo email address for example, it would (I think) let you login with a yahoo account instead.

The guy who founded this company is the guy who invented oauth ;-)

It's true and important why wouldn't it be enough?

Well, if the social login prevents him from signing up, he obviously can't even try the product. So how could he have any comments about the product itself, whether positive or negative?

Here's a less blunt version of @sixQuarks's complaint, which I think is completely valid:

Poetica has unnecessary friction in the signup process, and the very existence of this friction implies that the founders might not have thought long & hard about what kind of users they want to attract. IMO, people who write a lot and use Markdown to do so are more likely than the general public to be concerned about the privacy implications of social login and therefore wish to avoid it. Poetica also seems like the kind of product that could benefit from a quick online demo right on the landing page, but the founders chose to require signup first. Both of these factors are surely detrimental to their conversion rate.

Negative feedback is more useful than positive. Saying "nice site" tells the creator nothing. Pointing out a flaw tells the creator something possibly in need of improvement.