Hey Christina,
Do you have unique SFDC campaigns tracking conversions for each of the assets in your nurture paths? If not, this is a great way to report on what's working and what's not in your nurture programs and also allows you to compare performance of these assets in nurture vs. various other channels, especially in terms of responses and pipeline contribution.
If you want to get more granular, one good way is using custom fields on the campaign level in SFDC to hone in on key variables. For example, if you wanted to track performance by content theme and type, you could create custom fields for both on your SFDC campaigns for nurture assets, and tag each campaign accordingly. Then when you go to do your reporting, you might see that nurture campaigns that influenced or created the most closed won opportunities were those related to content theme A, and those that generated the most responses were content type: eBooks.
Hope this helps!
-Gary