Correct. Although NXT is 'diggable', but with pure PoS it just means an insane interest rate for the richest, and nothing for those who have nothing in the first place. Kind of fiat 2.0 without the FRB bomb maybe?
Mastercoin, though not minable per se, is not really a coin of its own, but a extra layer over the existing BTC block chain.
Ripple is not minable, but it's quite innovative in itself. Besides, the XRP in your Ripple account are not supposed to be your "Ripple wealth", theoretically you only need a few to push your trust lines to the ledger, and then work with any currency that you like (through IOUs of course, this is Ripple). Some Bitcoiners hate it mostly because of the proprietary stuff in it and it's bond with Ripple Inc, a for-profit corporation.
tl;dr: you'll have to be really innovative if you want success with a non-minable coin.