The change means that anything pated into messages is reduced to plain text. The change was made becasue too many users were just copying entire pages from other sites and pasting into messages. This causes all sorts of problems.

One impact is that images from other sites cannot be simply copied and pasted. Those images can still be inserted into messages but the user must right click on an image, then properties, then copy the image url. They can then use the unsert/edit image icon to paste the url into the box and that will then display the image in the messge.