The concept of something being visibly off about dru. You cannot describe it, or see it - but you can FEEL it. That her beauty, her smile, is very much like cracked pieces of something than once was whole and beautiful. That is the very first word that comes to mind when people meet dru: fractured.
Because she is, her personality is fractured. Her mind is fractured. I think i can best explain it as the notion that drusilla isn’t a whole person anymore, but instead she is simply fragments of emotions that she feels and experiences and take over violently, one at a time. Her brain, trying to protect itself, will not let her feel more than one emotion at the same time, therefore, and also thanks to vampirism, the emotion she experiences at a given moment absolutely consumes her. Let it be rage, lust, jealousy or obsessive love. Dru’s mind compartments her emotions because it knows she would not withstand feeling so much at the same time again ( as when she was being t*rtured by angelus ).