The severity stems from the unfortunate fact that a password leak retroactively, and silently, destroys your security across all sites that use the same or a similar password. Even if you started using the longest, randomised, two-factor-authenticated password system last year, all those forgotten or seemingly unimportant accounts are suddenly exposed.
Even when the exposed sites have minimal information or impact, minor information in aggregate adds up to a lot of danger for escalation and social engineering.
Now consider that there are huge swaths of people with the same password that they've use for email, banking, medicare, and everything else.
A proper response from Dropbox would be to explicitly and loudly inform every leaked email address (not just their current users) that they need to immediately change every password across any and all sites that might use the same leaked credentials.
Furthermore, Dropbox should set up a secure site with a unique link per email address that allows a user to key-in and check their memory against the exposed hash. I know that I have changed my password for Dropbox at least twice since 2012, but in 2012 I might have used an insecure password. Allowing me to figure it out before a nefarious party would allow me to better judge the potential personal impact.