From what I've heard from different friends of mine who have converted to Islam, there are a few main reasons for most conversions.
1. Islam is a religious lifestyle, not just a quick Sunday morning church visit and a label. Although some Christians do pray before each meal and spend much time with their Bible, etc.many don't in the USA. Islam is something that can't be shunted into a few spare hours. You pray five times a day. Your dress code is affected; even your food choices are explained in the Qur'an. Many converts like it because they feel actively involved and as if they are truly religious.
2. For women, it is a big and welcome change from the "in your face" sexuality of western culture. Women are not required by the Qur'an to veil. They are required to dress modestly and it is *recommended* that they cover their hair. However, most do veil and some go farther with niquab (the face screen) or other traditional clothes. Women dressed like this consider themselves marked as religious women, not easy dates. They don't get hit on. No one gawks at their bodies. They are clearly off limits sexually and many that I personally know say it is liberating not to have to conform to western standards of sexy dress, makeup and spending so much time and money on appearance. You buy a five-dollar hijab, put your hair in a ponytail and cover, wear long skirt and a tunic shirt -you're set.
3. Islam is also growing because many people are not satisfied with Christianity. They feel it is too liberal, too fluid and changes for the culture. Islam on the other hand is more rigid and does not as a whole allow much for modern changes and the whims of society. Many like it because they feel other religions have loose standards.
That's what I got from the new Muslimah's group at Yahoo, Islamway women's board and a few other friends of mine. Stats taken by many college groups say that women convert 4 times more often then men.