The campaign builder is an unmatched canvas for automating almost any business process imaginable. It doesn't restrict you to a single workflow paradigm, so you have the freedom to design systems to match your business AND organize them how you want. I love the options of using tags, custom fields, notes, and API goals for storing data on the contact record, segmenting contacts, or automating based on that info/event. Plus, all of this feeds a reporting system that can track any aspect of a business's growth and health (with some configuration). The extensive API and HTTP post automation supercharge all of this with custom development or addon tools.
The new responsive email builder is extremely user-friendly, and has a nice assortment of premade templates, as well as letting you build a library of your own custom templates.
Lastly, the Infusionsoft community is deep and extremely knowledgeable. Practically any question you have, you can find an answer to. Infusionsoft's own support is solid and generally satisfactory, but the breadth of experience available from other users is an incredible resource.
At some point, it becomes essential to start using "satellite" helper tools, like PlusThis or MyFusionHelper, to address some of Infusionsoft's shortcomings, manage data more effectively, build graphic reports, or create custom automations. This adds to the baseline cost.