Why does Electrum's SPV require proprietary servers?
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.