It's really annoying when someone answers a question like 'what is your fave tool?' and they write 4 in a bulleted list. This makes it hard to comment on the individual bits. It would be cool if you could drag over one of the answers, hit the 'split to new answer' button and have it become its own item.
Since this kind of question is tolerated at max, but definitely not encouraged, I don't think there's much to be done.
Also, this sounds close-to-impossible to reasonably implement UI-wise. If you have strong feelings about a particular case, you can post a new answer with one part of the source answer, and edit the source answer to remove the migrated part.
Since the Trilogy strongly discourages discussion type or list type questions, although to some degree some exists, this is really not a needed feature. Comment to the OP and ask them to break up the answer, and since these questions are meant to be CW, they won't have any rep gain either way.
I don't see a need for this. Anything to discourage users to post discussion or open ended questions I would welcome with open arms.
As balpha pointed out it will also be a pain to implement. What logic do you use? Split by line? Split by word?