Historically and semantically, SQL Developer is a developer tool. There is very little comparison with TOAD for the dba.
But… Oracle is working on this! In the latest release, 3.0 EA1, you will be able to see Tablespaces, Redo logs and controlfile, finally.
Still no session browser, but I am delighted that the database administrators are not getting forgotten !
Thanks Surachart for spreading the news!