I have been spectating some multiplayer games today. in one game i was afk 10 minutes and a card i had not seen popped up. After clicking the card away, the game played every move that had taken place while i was afk. Just like watching what you would call a replay. Wouldn't this be a cool feature? Who wouldn't want to share their most glorious battles in both singleplayer and multiplayer? And if tournaments become a reality, replays of the finals could even be broadcasted, so everyone would have a chance of seeing them. Not just the ones online at the moment the finals were played. Well just an idea. Don't know how hard this would be to implement or what the filesize of these replays would be.
I mentioned this in another thread, but I don't think it would be hard at all. They just need to make something that can read the game log we already have by pressing F1 like a script, and then maybe abbreviate parts to cut down on space. (For example, a program wouldn't need a whole card name; it would be able to use a three or four digit ID tag)
Having written systems like this myself, the recording of it isn't hard, may take a little digging/time to make sure every event is recorded though. Developing a system to replay it is a bit more difficult and time consuming.