Again, just to reiterate on the piece of my post you didn't address, I do not like the centralized nature of the Masternode setup at Darkcoin, BUT , many coins have a centralized aspect, such as checkpointing servers in some POS coins. the way i see it, masternodes and checkpoint servers may be a necessary evil. So all in all i guess the Masternode setup isn't a big deal, so long as it offers no unfair disadvantage to common miners.
The concept of center, is that it is singular. It is a single point where all the other things revolve around / are based upon etc.
Checkpointing is centralization because there is one point where the network uses to align itself.
Masternodes are like ...500.
500 is not a single point / a center. There is no "center". There are 500 masternodes all over the globe.
If there was one masternode that did the mixing, it would be centralized mixing.
If there are 500-1000 nodes all over the world that operate in a p2p fashion, it is not centralization.
If the government can go to one place (center) and take down the system = centralization / client-server model - with the server-mixer being the single point of reliance.
If the government can't do that because it has to shut 500 nodes all over the world = decentralization / p2p model - with every node adding to network resilience.