I think we all just want to see a real american (no, not just because Rupp is white, because he's a legit born-in-America American) fulfill the potential of being a world class (i.e. Olympic medal contending) marathoner that Ryan Hall showed (...but failed to accomplish).
American athletes have succeeded in being at the top of the sport in just about every event (track as well as field) from the 100m up to the 10k at one point or another in recent years (less so in the 5k and 10k but there have been good moments i.e. Goucher, Flanagan, Rupp). Marathon gold at Olympics/World Championships/WMM's has on the other hand been fairly elusive, hence why it is a HUGE deal when rare moments like Kastor and Meb in Athens or Meb winning Boston happen as opposed to when (insert name of Kenyan or Ethiopian athlete here) is successful in significant marathons which is to be expected and not much to talk about (kind of a debatable statement if you are an Ethiopian/Kenyan, I know).
^ Thus, people want to see Rupp get going on transitioning to the marathon because (among other reasons) he has proven to be capable of mixing up with/beating the East Africans on the track so the hope is that he can do the same in the marathon, an event that hasn't had much American success since like the 70's and 80's.