Different clients have different reasons for booking us. We book a lot from one of the local bridal shows, and those folks like our price AND our personality. This year, we were priced close to 2 other DJs but people liked our personalities. We weren't just trying to make a sale, we were genuinely interested in them.
There are other folks who read the content on our website and blog and think "right on!" so they email asking if we're available and where to sign.
Of course, the fact that we're close in age to the majority of our clients helps. They know that we know what's hip for tunage and that we keep up on the latest trends. However, we'll land older clients who just think that we "kids" are great, especially where we know our older music. (That's more the J Mac; he's a classic rock buff.)
And then there's the equipment. There are people who book us because we use passive JBL speakers. There are also the folks who don't want a laptop DJ, so the fact that we use CDs sell them. Our occasional usage of vinyl tends to attract appeal, too.