Yoda,
My concerns are about liquidity.
I generally trade stocks with average daily volume of over 1,000,000 shares, but not the extreme high volume stocks like DELL, CSCO, MSFT, etc. I prefer to be able to trade a stock that is somewhat readable for the short term with enough volume so that it is liquid, but not so much that the price won't move far very fast. Most of my trades only last about 7 minutes, though others, such as trending plays may last for most of the day.
When I place an order to trade 1,000 shares, most often, the order does NOT trade in one segment of 1,000 shares, but rather in multiple smaller segments such as 500, 300 and 200 or something like that. I primarily use REDI, ARCA and ISLD. (ISLD orders are most often in many pieces.)
My concern is that perhaps the traders on the opposite side of my orders are often from those with less than 25K, and they may disappear from the market because of the new 25K rule. I assume this since most often it appears to take several other smaller traders orders to match my 1,000 share order. I hope I'm wrong, but I see this as a disadvantage to the more professional daytraders if liquidity dries up.
If the statistics are true that about 90% of daytraders fail, and a large portion of those 90% disappear from the market because of the 25K rule, then many of us may be affected adversely unless you want to scalp for penneys on the high volume stocks, accept poorer entry price, or hold for longer time periods. I don't think that general stock price fluctuations will change, but I suspect that entry and exit will be more difficult if liquitidy dries up.
I guess we'll find out soon, but I hope I'm wrong.
TimWin