Currently the org.apache.accumulo.core.util.shell.commands.SetIterCommand requires that the iterator implement the OptionDescriber interface (line 128). This prevents using some useful iterators at the shell, for instance the DebugIterator. A nice way to learn how iterators work is to set this iterator, use "debug on" at the shell and start scanning different ways.
There are two possible solutions that I see. One is to make all iterators delivered with Accumulo implement OptionDescriber, and document this shell quirk, or to use reflection in the shell code to see if the iterator class implements OptionDescriber, otherwise just ensure it implements SortedKeyValueIterator.
