One reason I usually wait is that I'm a cheapskate. I hate to say it since a number of my friends are authors, and authors don't make bank on used copies of their books, but I almost always buy used.
On the other hand, it's often the case when I buy used that I'm simply replacing something I bought new: I lost it, or loaned it out, or read it until it fell apart. So the author made his buck the first time around; the second, he's going to have to settle for the fact that if I liked the book enough to buy it twice, I'm probably going to tell everyone about it.
For example, the new F. Paul Wilson novel came to $17.15, and I pre-ordered. Being, as I mentioned, a cheapskate, I decided I'd rather round out my purchase to $25 and get free "Super-Saver" shipping than end up popping $22 for that $17.15 book. Sooo ...
I've said it right here before, and I'll say it again -- if you haven't read Ken MacLeod's Fall Revolution cycle, you need to. I've read each of the four novels at least twice, and three of them more than twice. I was able to find used "fulfilled by Amazon" (and therefore eligible for that free shipping deal) copies of Fractions
I need to get all the F. Paul Wilson stuff in one stack and figure out what's missing and what I need new copies of -- I foresee a complete re-read of the "Repairman Jack" books leading up to the release of the finale (Nightworld). Actually, what I really need is a stimulus grant to hire a curator for my damn library.