Saying this as a recent entrant to the SLR world but one not entirely unfamiliar with it on entry:
Opportunity cost. Someone just starting out is not going to want to buy used because generally buying used means you haven't got the warranty support a new camera would have. That peace of mind is certainly worth something, especially after the sticker shock an SLR body can create. New also means you have that 7-28 days to return the body if it really disagrees with you - something again used cannot provide.
When it comes to controls, i'd tried friends' 50D and 500D. The difference in access to controls especially the dual control wheels vs single was nice. It wasn't a massive difference from holding the exposure button to get the single wheel to shift mode - like shiftclicking on a computer. There weren't many things that were just plain not possible on one but doable on the other.
Then I got to looking at the D3100, 5000 90 and 7000. The whole older lenses thing is a bit of a red herring. A beginner is not likely to buy used lenses for largely the same issues as buying the body. And given all of Nikon's new lenses have been AF-S for a while, it's becoming less and less of an issue with time. Even Canon's lenses despite their bodies having motors have most (all?) of their production lenses having in lens motors.
Image quality wise and this was by far the most important for me, the D3100 was only bettered by the 7000, a body nearly 3 times the cost. A cost differential that's allowing me to build a decent lens collection, with a nice 17-50, looking around for a 90mm macro/portrait lens and a decent zoom (though I'm waiting to see if nikon release a 70-200f/4 like Canon's - the 2.8 is beyond my means).
That aforementioned price difference gets you a lot of nice things - better low light, the in body motor, nicer ergonomics, sturdier body, some weathersealing, good burst rates. Of those only 2
actually affect the images you can capture (the frame rate and the low light, though low light can be compensated for by nice lenses), the ergonomics just make it harder to get to things rather than outright preventing something, and sturdiness and weathersealing - how many new SLR owners abuse their cameras, bearing in mind the lenses can't take the same kind of punisment unless they're costing as much or more than the body), and how many of you would take your pro bodies out in the rain unprotected?

The extra stuff the outlay gets you is nice if you already know what you want. If you don't, the value is in my eyes questionable. Which is why I got the D3100 now, got a nice 17-50mm, will get a 90mm, and a 70-200 (or 55-200 if it doesn't show soon).
Then at the end of the year I'll know what I want and where the kit
really limited me if at all and will be happy buying a used D7000 off the people upgrading to D400/D800/D4
