Any softfork that privileges or distinguishes miners from other nodes.

IMO this is actually the main thing that’s objectionable about BIP300/301, not the concept of Drivechains in themselves

Having a new type of message and payments that go to a miner “I want to start/merge-mine a drivechain” means you need to *know* who the miners are, and they need to handle the BMM in the coinbase tx. This is very different from all nodes equally and blindly passing transactions and blocks around to each other, not caring if the other end is a miner or not.

To me, it is critically important that people can mine without being (easily) detected.