Starting December 12th after the Wednesday downtime, replays will no longer be available for active tournaments in Magic Online. We realize that as long as replays are available during events, many players feel required to watch them, if only because they might gain some slight advantage otherwise missed. We feel like this behavior is detrimental to the overall experience and reduces fun, and this change is being made so players can focus on playing and enjoying Magic rather than studying replays of games at a breakneck pace between games or rounds.
As always, replays will remain available for viewing right after events have concluded. Replays for Premiere Events and Daily Events remain available for 12 hours, and all other events have replays available for 15 minutes after they end.
For discussion on this topic, please see this thread in the official Magic Online forums.