A few notes on this. You are still limited to a governance threshhold in your map reduce scripts. While you don't have to worry a ton about governance, you still need to do so when gathering your input data and doing processing. It is best to get all the data you can up front but if you have a TON of records, you can run out of governance depending on how you gathering your data. For example if you are doing large record processing, its good to search for all of your data and then combine it rather than doing lookups on a per object basis. Its also good to create your own custom objects rather than returning search results as your custom objects is going to be smaller than a search result object.