Another little detail that Hermann Baer mentioned in his presentation yesterday was the ability to create multiple indexes with the same column definition – something which currently gets you Oracle error “ORA-01408: such column list already indexed.”
No details, and there’s always the “safe harbour” slide of course – the one which says seomthing about the presentation being only an indication of current thinking and nothing is guaranteed to appear.
Having said that, this looks like an interesting option for those (possibly rare) occasions when you want to change a unique index into a non-unique index (for example, to change a unique constraint to deferrable). Rather than having to drop the index and create a new one – leaving the table unindexed while the index builds, you appear to have the option to: “create new index online”, “drop old index”. Moving a primary key constraint from one index to the other might not be so easy, of course, especially if there are foreign keys in place – but this certainly looks like a helpful step. [Update: actually it's easy to move the constraint - as I subsequently found in this post.]
Details to follow when 12c becomes available.
Update Sept 2013
Although you can create multiple indexes with the same column definition, only one of them can be visible at any time – so this should remove the temptation that Richard describes in his comment below. It won’t stop people creating “duplicates”, though, and leaving some of them invisible for a while just in case they need to change their minds. Always keep firm control of your indexing.