The percentages that are often cited (53%-47%) do not necessarily indicate a "bottom" 47% income level; it is just an indication of the percent of total tax returns that are non-taxable. Included in that 47% are 2.8 million that earned $50-75,000, 361,000 that earned between $100-200,000, and 4,000 that earned over $1 million. Included in the taxable returns are 157,000 that made less than $10,000 and 5.4 million that made less than $20,000.
http://www.taxpolicycenter.org/numbers/Content/PDF/T11-0226.pdf
It's mainly about your unique tax situation. For instance, a family of four that made $50,000 in 2010 could have this situation:
The family got a standard deduction of $11,400 and four personal exemptions of $3,650, leaving a taxable income of $24,000. The income tax on $24,000 was $2,769.
The two children younger than 17 qualified for two $1,000 child tax credits. The Making Work Pay credit was $800 because the parents were married filing jointly. The $2,800 in credits is more than the $2,769 in taxes, so they paid nothing.
But you know, these "loopholes" (I really dislike that word) were institututed to help families with children and people who work, two things that don't seem too subversive, and are provided to probably the largest "special interest" group in the country - working families. Most tax deductions and credits are to encourage behavior that most people would agree are worth supporting. So saying 47% are slackers that don't pay taxes on average is like having one hand in the oven and the other hand in the freezer but, on average, the temperature is moderate. There is often a different story behind the simple statistics.