Connecting William Street is a redevelopment project that the Ann Arbor Downtown Development Authority is working on to make better use of properties which are currently surface parking lots. The grand plan envisions relatively dense development on these lots.
For background reading see the DDA site for Connecting William Street, the AnnArbor.com coverage of Connecting William Street, and the Ann Arbor Chronicle coverage of Connecting William Street. If you're coming at it from scratch, expect to spend about an hour chewing on the debates to date. Or, you could be like most people and not really care to read every excruciating detail of every minor functionary's opinions on the project, and every excruciating opinion from the pseudonymous commenter crowd.
William Street is not a fun street to walk down, especially compared to Liberty Street one block over. There's not a density of retail that makes window shopping appealing, and two entire blocks are given over to parking. The question is whether municipal intervention in the planning process can enliven this area and at the same time generate a return on the investment in the land that's better than what parking can provide.
The biggest problem with William Street, and one that no easy planning process can address, is the horrible car-pedestrian interface at the Fourth and William parking structure. Cars enter and exit to the sidewalk without much in the way of sight lines to help avoid collisions, and it's a busy enough structure that if you go from Main to Fourth past the structure you're bound to have to make a quick step to preserve your good health. No matter what else you do with all of the other properties, there's still going to be a barrier that separates Main Street from points to the east, and anyone with a choice as to where to walk is going to detour towards Liberty.
Build all you want, but the giant hulk of a parking structure smack in the middle of your development effort is going to overshadow any attempts you make to build a pleasant streetscape.