Seamless integration with all other products.
I think this is the most important thing for product recommendations. Having a robust integration with all products should be the main focus, as it will allow for much easier cross-sell opportunities. I think the main area to focus on is the integration with experimentation.
Experimentation and product recs are perfect complementary products and I think we will see a huge volume of cross-sell/up-sell amongst these two products. Having a simple integration between the two products will make it much easier to sell.