What is the main difference between Halcyon and Darkcoin (and other anon altcoins) ?
I think that HAL would be best compared to DRK and XC, in that they have some variation of decentralized anonymization where the mixing/anon process is distributed to give anyone a chance of participating in the process. I'm not a programmer so I could not explain the details and merits of each of these three coin's systems (although I do believe they are very similar).
Compared to other coins, HAL, DRK, and XC stand out due to this decentralized system. I don't think other coins aside from these three, have incorporated such systems.
I believe what the HAL dev is going for is a HAL-based platform that incorporates a marketplace and other features that may make use of HAL's decentralized anonymous system. In this way, HAL separates itself apart from other coins in that it has built an "engine" in the form of the anon system, with planned features for add-ons, such as the marketplace previously mentioned. XC is doing a similar approach I believe, though I'm not sure what they're exact plan and process is like.
What I can say is that the HAL dev is a single dev, and in the 1 month since launch, he's accomplished and provided much more innovation, updates, and professionalism than 99% of coins out there. This by itself sheds an optimistic light on the future of HAL.
