If preferential inhibition of JAK1 can improve upon JAK1/2
Although in that case the Galapagos compound comes into play as well. There was some good discussion over on SI about the long-lasting metabolite of the Galapagos drug.
I personally have high hopes for a JAK1 drug in autoimmune disease. The basis for this is that inhibiting JAK1 in theory blocks STAT3, which in turn is one of the critical pathways for TH17, which I have come to believe is one of the key players in autoimmunity.