iTunes 11: Default Podcast settings
Right-click on any podcast and select Settings. Or if you are in list view, select the settings button in the lower-left corner:



In the Settings for: drop-down list, select Podcast Defaults, then select Download all and All episodes:



Make sure that all of your desired podcasts are using the new default settings by selecting each one from the drop-down list and checking that the  Use Default Settings checkbox is checked:



Update:
iTunes 11.1.2 adds a new dialog for changing podcast settings. Select Settings and select Defaults... to change the default podcast settings.

Update:
In iTunes 12.4.3.1 go to Podcasts, and click Defaults at the bottom of the list of your podcasts. Then you can set the default settings for all your podcasts (changed settings will also apply to existing ones if they're still set to default settings).