Why is it dangerous?
There are a few reasons. COVID-19 is a novel virus, which means that many of its characteristics are not fully known yet. Important metrics such as fatality rate, for example, aren't understood with any level of precision yet. This means that responses have to assume the worst.
What is known, however, is problematic.
First, the overall mortality rate for COVID-19, while not yet known with precision, appears to be substantially higher than this year's seasonal flu variant - approximately 1% versus a typical seasonal flu's 0.1%.
If those numbers are correct, COVID-19 would kill the same number of people in 1,800,000 infections that this year's seasonal flu has in over 33,000,000 infections.
Second, because COVID-19 is a novel pathogen, there is little certainty about its ability to spread. The early evidence suggests that each infected person is responsible for infecting around 2.2 others. It has infected more than three times as many people after 60 days than the coronavirus SARS did in more than twice that time.
Third, because COVID-19 has broken out worldwide in spite of virtually unprecedented efforts to contain it, and the extreme measures that were used in China are unlikely to be achieveable in other countries.