The whole concept of 'political correctness' has been an albatross around the neck of the liberal left ever since it gained currency, what, twenty years ago? The sad thing is that that old reactionary bogeyman, 'political correctness gone mad', is not without a grain of truth. Part of the problem, as you mention, comes from people rather piously and high-handedly getting offended on other people's behalf, often without bothering to find out whether any offence has actually been taken. Then you've got things like the photo of Brunel with his cigar Photoshopped out for the cover of a school history textbook, which is to do with influencing behaviour rather than trying not to cause offence, but comes from a similar place politically, I think. It may be a pretty trivial example in itself but I think it's worrying in what it reveals about the mindset behind it: deliberately falsifying historical fact in order to satisfy a modern-day ideological agenda. At the risk of sounding hysterical, you can't tell me there isn't something a tiny bit Soviet about that.
And now with media like Facebook and Twitter that encourage people to type the first thing that comes into their heads and broadcast it immediately to the world, the old right-wing whinge that "you can't say anything any more" seems to be becoming ever more justified.