What makes spam bad?

There are many reasons for which spam is bad.
To an ordinary user, spam arises problems with managing their e-mails, it is very annoying and costs time to clean up the mailbox.
A major problem with spam is that it creates much of the traffic congestion in the Internet (as seen on this graph from Distributed Checksum Clearinghouses website). This causes a real costs to ISPs and big companies.
Spam is disturbing for most of people, and for many reasons- depending on what role they play as victims of spam. Broader explanation on why spam is bad can be found here.