In a word, I'd have to describe Carmilla as boring. It ranks extremely low in my personal list of 'good vampire books'. I had the sense that the story was being rushed through and to me it was painfully obvious that a male author was narrating as a female; I don't want that to be palpable and noticeable when I'm reading, because it throws the atmosphere.
Like victauria, I found the mythos of LeFanu's vampire weak at best. I really didn't enjoy the way that vampires are presented in this book, and in generally I am extremely un-fussy with my vampires. Alongside that, the vampire herself struck me as vapid and stupid throughout the entire novel - now that's just not a personality that I can associate with vampires and it's one of the reasons that certain sparkle-pires set my teeth on edge in a very similar way.