w00t? A billion attributes on hundreds of billions of rows? Could you think more of an edge case? You're just coming up with shit so you can show off how l33t you are. If you have that kind of a need, just go for a NoSQL server, nobody is stopping you. This feature is a nicety to have, not a replacement of NoSQL.
You could also do it in pg pre 9.4. Just create functional indexes on the paths you wanted to index. The advantage of 9.4 is special indexes for jsonb, which may index the whole field or just some paths. So even pg 9.2 was better than SQLServer's approach...
You can do that only for simplest key-value json, not the more complicated ones IRL.
E.g. an array
ID, book_title, book_intro, book_tags
here book_tags is a json array. Now try index that!
Mongodb could index it, full text could index it, pg 9.4+ could, but not other RDBMS.
Support nested data structure in RDBMS is hard. You have to implement flat/unflat voodoo in a weird & lame, non-SQL DSL