The interface is dated and inconsistent. Icons are the opposite of intuitive, the sync application is buggy and error-laden with conflict resolution a constant, painful fact of life.
Security is immature without the ability to allow users to administer user access or manage workflows, but limit their access to sensitive documents. (e.g. a super user has access to everything).
You need an advanced degree to attempt workflow creation or modification and only a developer has a chance of succeeding.
Search is painful. For example, search results cannot be configured to display a full path, so multiple results in similarly named folders appear identical in search results.
Sharing is archaic and cumbersome. Few options exist for controlling how inside or outside links are managed, expire, controlled.
The Salesforce integration app is slow and unintuitive.
Overall, the system needs the help of a serious overhaul from a user experience perspective. Review collected by and hosted on G2.com.