Why does Electrum's SPV require proprietary servers? What does an Electrum server do that a regular Core full node doesn't do? If the wallet downloads block headers and verifies transactions using SPV and bloom filters, why does it require special servers at all?
Quoted directly from the source:


  One security advantage of Electrum is that servers are authenticated
  using SSL; this provides protection against MITM attacks, which is a
  problem for thin clients, including bloom filter clients. However,
  this advantage comes at the cost of  privacy, because the main
  Electrum server you use knowns which addresses you are using; bloom
  filter clients have more privacy although the default setting in other
  clients is to favor speed at the expense of privacy.