I'm just playing with dashboards beta (which is great, mostly).
A dashboard is something I want to set up and monitor, not run like a report. So the concept of reporting period becomes important.
For example, I want to monitor project costs over the last 3 months. As set up in beta, I have to filter by month, then select the last 3 months. After month end, I have to de-select the earliest month and select the new month. Every month.
So a criteria like "last 3 months" and similarly "last quarter vs prior quarter" would be very useful for comparing and seeing trends. If I want a detailed analysis I can run a report, the dashboard should alert me that I need to do so, without having to be validated every time I look at it.
Note that generally I look at things over short periods of time - this year vs last year has no meaning where projects that run for 3-6 months are concerned.