The commuter tax deduction lets you buy the cards with pre-tax money deducted from your paycheck, which is a substantial reduction up to 45% or so depending on your marginal tax rate. Of course, because it's using pre-tax money, it's giving a bigger discount to higher earners, which makes it the opposite of progressive, which seems opposite of what's intended.

Separately, seniors/disabled people pay half fare: http://web.mta.info/nyct/fare/rfindex.htm