+if [ "$DISABLE_HADOOP_CLASSPATH_LOOKUP" != "true" ] ; then
Please name this HBASE_DISABLE_HADOOP_CLASSPATH_LOOKUP. Not that I think we're likely to conflict on the name, but just in case.
Also, if you just check if the string is non-zero length it'll be easier to integrate programmatically determining if we should disable.
I think this is a valid use case where there is hadoop installed on the node(s), but we are trying to run HBase from a self-contained tarball. We should not try to find the "installed" hadoop in this case.
This is a very good point. How much more work would it be to get this exported when we're starting local distributed mode? At a minimum we should add a note to the ref guide telling folks to set it.