I have a huge soft spot for characters who are brave, sometimes to the point of foolhardiness; who act without stopping to worry about the consequences and with a certainty driven by a clearly defined world-view. In other words, characters who are nothing like me. *g*("Wait, wait, let's stop and think about this!") My #1 reason to love Cally Tyrol is that she was so frakking brave in standing up for herself and for others, regardless of how daunting the circumstances.
Whether she was biting off her would-be rapist's ear or telling Boomer where to put her sympathy, Cally was a mix of recklessness, determination and indomitability. Of course, some of these acts were easier to cheer than others, but they were all coming from the same place. And Cally didn't only stand up for herself but for her principles as well; that humans shouldn't turn on each other, that the working class deserve an equal opportunity and that there are certain things you just don't do, even to Cylons. Her faith in her own world-view could be limiting at times, but it also gave her a strength that characters like, for example, Kendra Shaw lacked; if Cally had been aboard the Pegasus Cain would have had to put ten of her against the wall, because there is just no way she would have - or even could have - collaborated in shooting civilians or gang-raping prisoners. Her stubbornness was her weakness, but also one of her greatest strengths.