I have found its switchover functionality to be most useful using which you can perform disaster recovery within no time. Its feature of FAL is also pretty useful that helps to keep both sites (primary and standby) synchronized. Logical standby database is also a pretty handy feature for reporting purposes.
Active Data Guard has helped us a lot in achieving the real time reporting by offloading our heavy reports from Production to Standby database.
Limited functionality to exclude any schema/table from replication would be helpful.
Currently Data Guard replicates whatever DML operation is performed on primary database is replicated to standby database as it is. To improve it bit further, if Oracle can provide some kind of feature in which user can select any schema or 5-10 collection to tables in some schema that user doesn't want to replicate. Because this kind of feature is available in Oracle Golden Gate which a separate licensed product with a lot of features like that one that I mentioned for limited replication, heterogeneous replication, cross database replication.
