Besides the obvious (such as projecting a sub-datum from a json datum), you can also do data transformations (whatever javascript can give you) -- as a contrived example, the sine of a number or the sum of two numbers -- and, you can try projecting arrays or hstores from a json and then using GIN/GiST to do a more expensive but more powerful index over them.
Some more exotic built-in indexing will probably show up in the coming years, but a lot of things can be accomplished relatively soon with this extension.
that's right. plv8 works. but it certainly does not feel like using a postgres index but a map-without reduce-framework. so i hope that built-in indexing will come sooner than later... :D
