#2 The form of key players

Several key players failing to deliver is what mainly led to the awful form of Borussia Dortmund earlier in the season. The likes of Sokratis, Marcel Schmelzer, Pierre-Emerick Aubameyang all failed to impress under Peter Bosz.
Although it's too early to make a final judgement call, all of these key players have looked significantly better under new coach Peter Stoger. Sokratis was rock solid in defence while Marcel Schmelzer was perhaps the best BVB defender in the game against Mainz.
Pierre-Emerick Aubameyang also picked up a crucial assist and was very good overall. Even Shinji Kagawa and Jeremy Toljan looked particularly impressive on that night, which wasn't the case with the Dutchman in charge.
The fact that all of these important are finally players to their expectations only bodes well for the club as they seek redemption in the second half of the season.