I'd like to see basically a letter, with a detailed list of the forms they had in the prior year (bullet point form W-2s, 1099s, 1098s).

Maybe a brief (undetailed) bullet point list of the type of expenses they had in the prior year - charitable contributions, real estate taxes, etc. No specifics, though. I don't need my 80 year old client that gave $20 to 30 organizations to have that on this letter/organizer. 

For most clients this should be one page, in not too small of a font.