...
This design is easy for the first-time user to grasp because it concentrates on presenting only the bills to a user. All of the links are underlined, which gives them the proper affordances. Similarly, the timeline is implemented as a scrollbar, so users will understand how to manipulate it. All of the links are large, and buttons are likewise visible. This means that the user will be accustomed to dealing with similar interfaces, and this will make the implementation easier to manipulate for the first time.
Visibility
The site presents information to the user that is not visually cluttered. The user's locus of attention on their main User Page directs them to the most pertinent information: when bills are due and how much the users owe.
Efficiency
Error Prevention
...