Actually smallpox is one of the more contagious diseases, with the patient being contagious before they have gross pustules, and the residue (ie: dried crusts) being infective for some time afterwards on surface (see the use of blankets of smallpox victims used as "gifts" to native Americans to spread disease in early colonial period). There is airborne spread. Prior to 9/11 essentially nobody had received smallpox vaccinations since the 1970s - and these become ineffective after 10+ years, although those who have been immunized to retain some residual immunity and/or get less ill if they do get the disease. Therefore on 9/11 essentially none of the US population under 25-30 had ever been vaccinated, those who had would have a weak response, and there was NOT enough vaccine in the US to vaccinate more than perhaps a quarter of the population, and this was old and its efficacy was somewhat in doubt.
Smallpox has an approximately 30% death rate in those with no previous immunity ("immunologically naive") and about a 5% rate of morbidity among survivors including blindness and permanent lung damage (skin scarring not included). Smallpox patients require a significant amount of nursing care, even if they are among the 70%+ that survive.
For an idea of how infective smallpox is look up the case in Germany where a "hippie" German imported a case from Asia, and people in the hospital on DIFFERENT FLOORS were infected due to airborne spread through the hospital ventilation system (while "isolated" the rooms air system was not Bio III).. Also reference the NYC experience in the late 1940s. In an environment like a subway at rush hour or an airplane one would expect significant spread. Another factor is since there has been no human smallpox since ~1973, and none in the US for some time before that, I would expect that recognition of the early stages of the disease, as opposed to the full blown manifestation, would be delayed - exposing even more people.
Using quarantine, containment, "ring" vaccination and vaccination of medical personnel, military, police, fire, EMT etc with available supplies you would not see a pandemic, but expect massive disruption, potentially many ill folks (and deaths), and lots of political disrutpion or worse over "who gets the vaccine."
BTW nowadays between folks receiving chemo for cancer, patients with Rheumatoid disease on methotrexate & similar drugs, and of course folks with HIV there are tons of people walking around immune compromised compared with the "old days" & they will be very hard hit.
