Ninjarette has brought up a lot of good points.
Personally, the term "nappy" only offends me when it is used in a way mean to insult me.
The funny thing is, I take it differently depending on if the speaker is relaxed or natural. I don't mean to, but it's like when white people say the "n word" versus black people saying the "n word." It's kind of like "only family can talk about family, so only natural girls can call me nappy because I feel I know exactly what they mean." Silly right?
One day soon, people will put the negative use of nappy behind them!
I also don't refer to my own hair or anyone else's hair as nappy. I don't know the condition of someone else's hair (whether it's tangled or unkept) because things can LOOK one way, but actually be another way. I just know how I like MY hair to look and feel to ME.