If you delete a Master-Detail Relationship, it will revert to a Lookup and you can expect the following repercussions:
1. You cannot easily revert the field back to Master-Detail., unless all child objects have a specified parent.
2. Child records will no longer be automatically deleted when the Parent record is deleted.
3. The security of the custom object will have to be set as it will no longer inherit the security of the parent.
4. The report type that was setup upon the master-detail relationship creation will no longer be accessible. You can get around this by building your own Custom Report Type, however all of the reports that were run with the prior report type will no longer be accessible either. When you try to access a report that was run with a report type that is no longer accessible, you will receive an error that reads: "Obsolete Report". This goes the same for if you change a standard look up to a master-detail
5. Roll-up summary fields on the Master object that references the Detail will need to be deleted in order to change the field type to a Lookup.
6. Cross object field updates will no longer be available for workflows