Of the 60 who were sick enough to need hospitalization, 7 have died. We do not know how many people were exposed-but-immune and how many got mildly sick and never saw a doc about it (and thus were never reported).
This is the best argument in favor of a lower mortality rate. We do not know enough to answer this question at this time, but the experience at Prince of Wales Hospital may be a clue.
A good portion of the staff got SARS. However, there were NO reports of mild illness in the rest of the staff. It seems unlikely that the disease would not have made at least some people mildly sick, but the lack of reported mild illnesses would seem to indicate that people in this category were not prevalent.
In a related item, the U.S. CDC reports a case of a person with a mild case of SARS, not thought serious enough for hospitalization, who nevertheless infected others. These others were made seriously ill.
The CDC cited this as evidence in two key areas. One is that the disease did not seem to be burning itself out, since mild cases would not cause severe illness if the mildness of the carrier's case was due to weakening of the virus. The second point made by the CDC was that the case provided evidence supporting quarantine of those suspected of being possible carriers.