The market design is unfavorable to retail investors and traders partly due the regulations and laws in the US.
- flow internalization which reduces liquidity on venues and widens spreads
- dark pools which is liquidity only available to institutions, therefore removing liquidity from venues that retail trades on
- market fragmentation which harms retail investors who don't have the execution sophistication of HFT or execution algos
- short sale restriction which harms retail traders who incur excess slippage
- allowing market makers to naked short sell but not retail investors which harms retail investors because they have to pay rents to access it
- trading halts which benefit institutions (over retail) who have the execution sophistication to handle unhalts and who can price warrants etc better while the product is halted
