What's the general atmosphere of the event? It's intense (lots of content), and no-nonsense. Every speaker had something valuable to say, and they didn't talk down to the audience at all.
I found the day motivating, especially DHH's talk and just chatting with my peers. With the exception of one person, everyone I met was a hacker and was working on something.
Is it similar to other conferences you've attended? Not really, one big difference: the overall quality of the attendees. I'm a shy person, but there were so many people talking about writing code (vs. big idea people) that I felt like I fit right in.
Is it simply a day-long sales pitch for YC? The only real pitch-man the year I attended was Jeff Bezos, his talk was a pitch for AWS. However, he was so impressive during the Q&A that I can forgive him.