Like many others in here; I find lots of appeal on Kisaragi's hair. Brown hair has always been one of the more appealing hair colors to me; and she's also often portrayed with a variety of hairstyles; all of which I really like.
Her big, brown eyes make for some really cute expressions too.
However, no talk about the appeal of Kisaragi could be done without mentioning her glasses.
They're of the round, "so big you wonder how they even work" type; which is something you don't really see in anime as much anymore, and also kind of like her signature accessory that distinguishes her from so much similar characters.