The first one is that we may not develop any meaningful immunity and a vaccine will prove elusive. Add to this equation that those previously infected might even get it worse the second time, particularly if they have lasting damage from the previous illness. Then things could get pretty bleak. Under this scenario this is not just going to kill a lot of the old and infirm but could affect a lot of others, not just in terms of people dying but those left disabled.
From what I've read, there is definitely not unbridled optimism within the immunology community that a vaccine is likely to be effective, neither does it seem remotely likely that exposure to the virus will confer full immunity, at least not long term. Therefore, unless we are prepared to accept the end of society as we know it, at some point we're going to have to accept that this virus is something that we're going to have to live with. The additional risk is minimal in the vast majority of cases.
Tens of millions of people will die over the coming years unless the virus becomes significantly less virulent. Most of them would have died in short order of another cause. Of course, some of the dead will be hitherto healthy individuals in the prime of their life, some will be children. There's no point burying our heads in the sand and pretending that this isn't a likely scenario. However, the consequences of shutting down economies and driving millions into poverty over a virus which has a mortality rate that by all reasonable accounts is far lower than 1% are simply unthinkable, and the product of a hysterical and catastrophic media driven overreaction. There will be a shift in the balance of public opinion. It's just a matter of when that shift occurs, but it will be sooner rather than later.
Edited to add: PilotLZ above was writing at the same time as me. His (or her) points are an excellent synopsis. Ultimately, life is not a risk free event.