Mostly the fact that they are equals. Damon would actually respect her because Bonnie is the kind of person who won't take crap, and Bonnie would be completely loyal to him, so Damon would finally have someone who loved him and only him. Oh, and there is their amazing chemistry, hot eye-sex, and blatant UST ;)
That they fit together, that they are so alike and yet different and I love when they work together, they are not vrienden yet but when they get there their friendship will be truthfull, honest and caring...
In the show,because Damon somehow used Bonnie and now she is taking revenge.And the fact that Bonnie is the only person that is not frightened door him,even door his bad side,is intriguing him(srry for my English).That's why he finds Bonnie interesting,and he enjoys watching her turning into a meer Damon-ish person somehow,meaner.And this love/hate relationship is aboslutely intriguing to watch,and u just stand on brand watching their scene,because everytime they look at eachother,you can see milions fireworks that burn your screen.
Books- Damon fell for Elena because she looked like Katherine, but he saves Bonnie before he even met Elena and liked her for her. A guy who has seen hundreds of girls and been around for years.
Show- They both have a little soft spot for each other even though they have every reason not to.
Damon and Bonnie are fire! They sizzle. She is strong and equally matched with Damon. They complimet each other. With relationship development they would have an epic romance.
I like bamon chemistry in the show. They always fight with each other but also help each other . They dont like each other but destiny always team them together for something . I think damon starts caring for her because he saves her in 3x21 ep but he dont give true explanation for this.