I work in the public school system and hear often from fellow elementary teachers that macs cannot open pc files. That is a really hard one to explain to people who do not understand that it is application to application.
I also hear that there is no software for macs. There is so much out there. Lately, I have been really into these little programs (like image well) that do one thing very well without bloat.
Also, one assumed myth is that macs behave and get viruses and spyware just like a pc. I am amused by people's responses when I tell them that there isn't anything to worry about on a mac. I get these looks of disbelief - hahahaha.
Most people only know pc's and windows. They aren't aware of different operating systems and when presented with a choice assume that they are all like windows, or should be. I think the general population is technologically immature. Hopefully, there will be more operating systems to choose from in the future, with more open standards among them. But I digress . . . .