If you attempt to dump a database from a 3.0 instance with MMAPv1 storage engine with a 2.6.6 mongodump, this succeeds. However, switch the 3.0 instance over to WT storage engine (same data), and the mongodump proceeds, but returns immediately without actually dumping out anything:

./mongodump -d test -c data
connected to: 127.0.0.1
2015-02-05T17:31:13.111+0000 DATABASE: test	 to 	dump/test

There is no error, the dump/test folder is just empty and the command exits immediately. Testing the same instance with 2.6 mongoexport succeeds, and data is returned, so this seems to be specific to 2.6 mongodump (3.0 mongodump also succeeds).

We may need to require mongodump >=3.0 for use with wiredtiger rather than attempt to make the older tools work, but there should be an error advising of that fact rather than a silent failure.