Leland,
Great topic for a thread.
I think the main obstacles limiting the profits of successful daytraders are as follows:
1) Liquidity. Obviously, a trader that trades in 100 share lots will not have trouble with moving the market with their orders. However, as the capital under management grows, the trader is often required to take larger position sizes in order to employ the higher capital available. Trading in ~5000 share lots or more can seriously impact a traders profitability due to the slippage caused by the lack of liquidity to trade in this size. The share size that begins to impact profitability will, of course, depend of the stock traded. Some stocks will allow trading in 5000-10000 share lots without liquidity problems, while others have liquidity issues at while trading only 500 shares or less.
2) Successful trading strategies. A successful trader typically relies on one or more profitable trading strategies, which are used for the basis of most trades. As the capital under management grows, the trader may simply run out of trading opportunities, as he/she is already acting on every valid signal in every stock, to the maximum that liquidity will allow. Once at this point, the trader is prevented from increasing the level of capital being managed until he/she finds additional profitable strategies which can be used.
3) Brain Power. As the capital being traded in increased, the trader must be able to simultaneously monitor a much larger number of positions. Depending on the tools available to the trader, this can reach the upper limit on the traders brain resources (or sanity), which can limit an increased trading level (and profitability).
How can these issues be overcome? Some of the largest (by capital, not waistline) daytraders I have met generated annual returns of $10-20M per year, at least in the glory days of the late 90's. They were able to handle the large size by trading mainly baskets of stocks, which allowed them to press a single button and buy/sell dozens of stocks simultaneously, totaling tens of thousands of shares. By trading baskets of stocks, the trader is able to simply monitor the basket and avoid most of the above problems. I suppose the same concept would work by trading the extremely liquid QQQ's or e-minis.
Other traders (such as myself) seek to automate their trading strategies to eliminate the brain power limit. This still subjects the trader to the liquidity problems, but by using automation the computer can enter and track positions in numerous stocks simultaneously, with each position size limited subject to the available liquidity level in that stock. Once new strategies are successfully implemented, the task of the trader is to constantly refine and improve the existing strategies, as well as to develop new strategies. For an automated trader, the real limit on trading profits is based on his/her ability to identify and implement an increasing number of profitable strategies.
I think the number of independent daytraders making profits in excess of one million dollars per year is probably very small, perhaps less than 50. I would say that the number is likely to be much larger than most people think, as successful traders typically have no reason whatsoever to publicize their success.
Best of luck,
-Eric