In answer to the original question, Viruses and Trojans, in my opinion.
I think that wifi per se is over-rated as a threat. In order to break in to your PC, the miscreant has to defeat any and all security at the wifi level, then actually connect to a PC, defeating any and all account security as well as any local firewall security.
Of course, if there is no wifi security and no account security (or using a 16-bit Windows), then access is trivial. But then anyone who leaves a home network in that state is unlikely to have implemented any AV or AMW software either. And frankly deserves what hits them. Unfortunately, they are also unwittingly responsible for attacks on other systems, as their infected PC is used as a zombie.
I have said before that I believe that the time will come when there is a requirement to demonstrate a level of PC competency and have 3rd party insurance to be able to connect a PC to the internet - just like a driving license and car insurance.
SD