The ability to use only one or two plugins for each section of our website, which makes customization easy! There is no way I can think off right now that would make me dislike it as much as i did when trying out other products in their category (such as Leadpages).
If you are looking at getting started with marketing automation then this should be near top consideration level product because its so simple yet powerful system once set up properly!! We were able manage all aspects from lead management through email campaigns & list segmentations very easily while also being highly customizable within those sections.