Although the forex market is by far the largest and most liquid in the world, day traders have up to now focused on seeking profits in mainly stock and futures markets. This is mainly due to the restrictive nature of bank-offered forex trading services.

There are many advantages to trading spot foreign exchange as opposed to trading stocks and futures. Below are listed those main advantages.

1. Bid/Ask Spread rates

Spread rates have tightened dramatically in the last years. Most online forex brokers offer a spread of 5 pips on EURUSD which is the most widely traded and liquid currency pair.

In the futures market spreads can vary anywhere between 5 and 9 pips and can become even larger under illiquid market conditions (which tends to happen substantially more often in futures currencies).

2. Margins requirements

Usually a foreign exchange trading with a 1% margin is available. In layman's terms that means a trader can control a position of a value of USD 1'000'000 with a mere USD 10'000 in his account. By comparison, futures margins are not only constantly changing but are also often quite sizeable. Stocks are generally traded on a non-margined basis and when they are, it can be as restrictive as 50% or so.

3. 24 hour market

Foreign exchange market trading occurs over a 24 hour period picking up in Asia around 24:00 CET Sunday evening and coming to an end in the United States on Friday around 23:00 CET. Although ECNs (electronic communications networks) exist for stock markets and futures markets (like Globex) that supply after hours trading, liquidity is often low and prices offered can often be uncompetitive.

Futures markets contain certain constraints that limit the number and type of transactions a trader can make under certain price conditions. When the price of a certain currency rises or falls beyond a certain pre-determined daily level traders are restricted from initiating new positions and are limited only to liquidating existing positions if they so desire. This mechanism is meant to control daily price volatility but in effect since the futures currency market follows the spot market anyway, the following day the futures market may undergo what is called a 'gap' or in other words the futures price will re-adjust to the spot price the next day. In the OTC market no such trading constraints exist permitting the trader to truly implement his trading strategy to the fullest extent. Since a trader can protect his position from large unexpected price movements with stop-loss orders the high volatility in the spot market can be fully controlled.

Equity brokers offer very restrictive short-selling margin requirements to customers. This means that a customer does not possess the liquidity to be able to sell stock before he buys it. Margin wise, a trader has exactly the same capacity when initiating a selling or buying position in the spot market. In spot trading when you're selling one currency, you're necessarily buying another.

Recommended further reading:
Origins
Who participate in Forex Trading?
Main Forex Markets
Market Dynamics
Speculation/Investing
Margin Trading
Trading with a Strategy
Risks inherent to Forex
Forex abbreviations