My father died last June and apart from the initial shock (which was mostly me not thinking about the fact that he died), the biggest thing was that he's no longer with me. I used to visit occasionally, and we'd talk for hours about stuff, but that will no longer be possible. The past will always be memories, but the sadness comes because we won't have any interaction in the future. The whole idea of him won't evolve, and you will no longer discover anything new about him (people talking about him don't really matter). What makes me sad is all the little stuff that will no longer take place because he is just not here.
But I'm used to the idea of everyone having an unknown, finite time on earth. So while the loss can be huge, I try to make every moment I spend with my family, positive and meaningful.