Customizable Contact/Organization Views
When I look at the Contacts or Organizations tabs, there is a very defined set of values that are listed, and this cannot be changed or customized in anyway. Turns out I don't really care about the Org's address, but I do care about some of the Custom fields we have entered for them, and I want to be able to see those on the fly without making a huge number of tags. Almost to the flexibility of a spreadsheet.
When evaluating CRMs, we looked at Nutshell which did have the feature, and it almost made us pick them over Insightly.