The new "Would you like to use another Repel?" feature, the Medal system, PWT for going above and beyond the gym leader rematches that I hoped BW would have but lacked, and the variety of Pokemon from different regions in the Unova Pokedex.
The gym remodels, as well as the personalized remixes of the gym leader building theme for each gym were great as well.
And basically every other little detail and addition they made. BW seemed so lacking in content to me when I played it, but BW2 added so many great features that they're honestly some of my favorite games in the series.