We've encountered multiple instances of block corruption (not our fault) where the mongod refuses to start. It would be nice to be able to "mark" a block as corrupt and have some tooling available to us to "manually" bring just the corrupt block in either via file copy or some other means. If it's an index block, it would be nice to be able to simply tell mongod to kill the index and rebuild it. Either way, it would be nice to have an alternative to "nuking" the entire database and performing a full resync for a (presumably) single corrupt block.
A customer just experienced an issue on a QA DB where they encountered a checksum error. It would have been nice to be able to bring the database up by having it ignore the "bad" block. Right now, they are in the process of running a repairDatabase.