In what ways is IEX different than a typical dark pool or a typical exchange?
You've said what's different in your question. There's 330 microseconds of network latency between IEX and anywhere else, so HFTs can't get information about trades in progress on IEX and use it to jump in ahead of those traders on other exchanges. All exchanges should have artificially induced latency of this kind so that if a trade is submitted simultaneously to all exchanges it reaches the furthest away one before a response can be received from the closest one, thus preventing HFT techniques.