The way to change your default template is: Go to Options > Files > Template Settings > Default Template Change it from Acad3D.DWT to Acad.DWT
Or you can do this by going to Options > System > Enable "Show All Warning Messages" and Click OK and then Restart AutoCAD, When the dialog shows up asking for the workspace. Choose 2D and you'll be set. If you know for sure you *always* want that default, then you can enable "Do not show me again".