Big agree on both the questions and the attire. I've served on hiring committees and people who ask informed questions give the impression they've bothered to look into the situation thoroughly--and were interested enough to do so. Even if you're pretty sure you know the answer to the question you ask, the interviewer won't know that.
As for the attire, I'm a fairly casual dresser myself, but I'm baffled at why people show up for job interviews in outfits that make me turn to see if Clinton and Stacey from What Not to Wear are following them in the door. If you really don't know what you look good in, find someone to take you shopping. And for God's sake, buy a color. If I have to see one more sad, ill-fitted black suit with a white blouse and clunky shoes on an applicant....Unless you're applying for a job as a nun, that's a bad choice. In fact, I went to a Catholic college, and ninety percent of the nuns dressed better than that. People are going to have to look at you all day long. Try to look both professional and pleasant. I don't think women have to wear skirts and heels, but they could wear dressy flats and a nice jacket or sweater with pants. And guys could wear suits or jackets that fit (and if they're heavy sweaters, they should invest in some anti-persperant, as no one wants to look at pit stains).