About Bancor: is it possible to trade any ERC20 token via the bancor project and smart contracts like it was possible on etherdelta (just by knowing the contract address one could trade the token, even if it wasnt listed officially) or only those that bancor has officially announced? If so I would definitely use Bancor from now on, because I want to support this great project.
No, it doesn't work like that unfortunately. New projects that want to be listed there need to deposit their tokens as a reserve, and Bancor has to add them to their network. So this part of their exchange isn't decentralized.
Alright, now as you said it, I remember something like this of Bancor. You are right, this part of it isnt decentralized, however the actual exchange process is, one needs to keep in mind though that they theoretically still could interfere with their tokenchanger smart contracts am I right? This would classify Bancor as of now, as a semi decentralized exchange, where the centralized nature could still impact the exchange if anything goes wrong (unfortunately also if someone would have malicious intentions).