I felt the same way after my first interview. I think part of it is that it is the first interview, and the whole thing is genuinely bizarre. see how you feel after your second, I think a reason I felt that way about the first interview was because, while it was a nice program, and I have nothing bad to say about it, it wasn't right for me looking back and comparing to some of the other programs I've been to. I guess, they've been doing this forever, and have a sense generally to figure out who they want pretty quickly based on the half day. Also, as the PD at my last interview said: all of you are qualified to go here, so really, at this point it becomes to some extent, a self-selection process, we have to rely on you to know where you will fit best.
and in my experience, the more competitive programs have asked me questions about myself, or questions i ask lead to questions from the interviewer.
The last interview I went on, I really liked, but none of us really got any face time with the PD, and that made me really uncomfortable, because it will most likely be in my top 3. on the other hand, another one of my favorites the PD interviews EVERYONE, and i think most walk away feeling like they got good face time and PD likes them, i don't know which is worse... any thoughts on that?