If you mean day-trading for your own account, there's nothin...

If you mean day-trading for your own account, there's nothing wrong with it at all. I will say that, from my own observations, the people who do it the most successfuly have similar techniques:

They track a very limited number of stocks, usually not more than four or five.
They became extremely familiar with the trading patterns of those stocks and were very good at recognizing buy and sell patterns.
They are highly disciplined in their trading habits — they weren't too greedy about deciding when to sell positions, and they weren't afraid to take a loss but knew how to limit their losses.
They understood the rules of the game they were playing — they weren't investors or speculators, they were traders. It's a very different method of operating in the markets.