I think an important option (Marketing people will LOVE YOU !!!)
Is to give the option to report, set, store the lead stage duration.
Just like the oppty report, the same for Leads will assist on learing the bottle necks and strong points in the lead conversion process.
I think that adding the number to the history report, is the first, and easiest step.http://salesforce.stackexchange.com/questions/24949/how-to-create-a-lead-aging-report