-Market making style scalping, buying the bid and selling the offer. The HFTs have monopolized the spread for the most part and this strategy doesn't work for human traders anymore
-News repricing. By the time you look at the news and check the chart you frequently will see a huge bar in one direction or another. The easy part of the move is now captured by the HFTs for the most part. You can bet on continuation or reversal but its a harder strategy with a worse sharpe ratio
-Share-class/ADR arbitrage, they took over that too.
-Repricing stocks when a big buyer/seller is detected. HFTs extract extra $ from these big players, in the past a lot of that $ went to day traders. It is still profitable but not as used to be because they are good at that
What I want to know is why small cap stocks which used to do their own thing now have such high correlation to the general market. Is this HFT too?