The most notable change is the new name: it went from FCKEditor, which many people didn’t like because of its similarity to an English bad word (it was made by the initials of the project founder, Frederico Caldeira Knabben) to CKEditor where CK stands for “content & knowledge”.
But obviously this is not the only change: it’s amazingly fast, it has a completely new UI, no more popup but only js modal dialogs, produces valid XHTML code (and hopefully it will not screw-up the code snippets written with WLW) and much more. Read the official release note to know more: CKEditor 3.0 is here!
And also I recommend playing with the demo for more complete experience. It’s impressive!
They say there will be a trimmed-down CKFinder with CKEditor 3.1 but until then, don’t expect to see CKEditor integrated with Subtext.