One is that the seller loses on additional value. The other is that the HFT has bought from a seller who would have sold to you, but sees a higher buy price and sells to that instead, so you miss your trade.
For low-volume traders these aren't huge issues, but for institutionals or others looking to make or exit a large position the costs can mount. Again: HFT wouldn't be undertaken if there wasn't value to be extracted in doing it, and that value is coming directly from other buyers and sellers, just as when you introduce a middleman to any other transaction.
It's the same kind of automation we've seen in many other industries.
That's ... an unsubtantiated assertion. Lots of evidence to suggest the situation's otherwise, at least in large part.
There was an excellent ACM paper posted to HN a few weeks back: