Darkcoin showed promises until developers failed to deliver what they promise.
I thought it was the opposite. The price surged when the developers showed that their DarkSend technology was actually working.
Now the boat has sailed, any new coin that uses the concept of mining (an alternative to this wouldn't be an alt-coin, it would be a new paradigm all together) will always have trouble getting of the ground.
Well, there are non-mineable proof-of-stake altcoins. Like Nxt, for example. Peercoin was the first PoS coin, but it was (and still is) secured by proof-of-work mining in the early stages.