Women love being able to tell if their man will be a good dad to their shared children in the future. Not all of us are looking to have kids, but many of us are, and it's a task you don't go into with someone who isn't going to do a good job. Sometimes, it's easy to tell. For example, if the man in question already has his own children. Being a good father is definitely going to be attractive to the woman you're with.
If you're not already a dad, the general way that you interact with children is going to be something the woman you're dating notices. If you have any small nieces or nephews or if she has young kids in her family, being part of their lives in a positive way will show her that you've got great dad potential.