Provide ability to create swimlanes, or exception workflows
Custom kanban board design functionality such as what LeanKit provides allow board designers to create horizontal and vertical splits within columns to allow for more complex workflows. An example would be a horizontal swimlane for work that needs to be expedited.
Leankit also offers excellent reporting features and can easily call attention to bottlenecks and backlogs by highlighting areas with work in progress greater than a chosen set point. A really stellar tool. Keep your eyes on LeanKit, Microsoft.