I had no idea what camera to get, nor how I would take to digital photography. My background is in SLR before I had children. With a completely non-technical wife (I've JUST got her on to email) and little personal time I had to weigh up how she could have access to our digital family history and how much time I would have available to make the photographs accessible to the family. My main fear was taking thousands of snaps that just stayed hidden on a computer to which only I have access. The cheaper option of the D3100 seemed less scary and less of a commitment to the genre.
Once I bought a large digital frame on to which I copy all my photos and put that in the kitchen so that it can just display random shots she buys into the access point. I got a Flickr page, onto which I put my "better" shots. We now harvest that to give framed and mounted prints to friends and family as presents (first outing this Christmas) and we intend to hang a personal selection up the stairs.
As I bought more lenses and got into macro and deep space object photography I started to suffer the limitations of the 3100. The onboard motor of the 7100 helps focus my 18-300mm, something my 3100 was struggling with. Focus is a big bug-bear of mine, so the better greater number of points of the 7100 was attractive. I love time lapse, and the 7100 has this in-built (I'd added it to the 3100 but couldn't really understand it due to a crappy manual). The 3100 had a 30 second Live View window which is REALLY annoying when doing things like astrophotography. Nikon have not released 3100 APIs into the wild so there are little third party add-ons available. The 7100 is the opposite. I like to go out in all weathers and, although not advisable, the 7100 is at least weather-proof. I now want to get into all the aperture, shutter, ISO stuff and the 7100 makes it all the more accessible as it's not all menu driven. My eyesight is failing (age, not illness) so touch controls are important.
I could go on, but a very good request BackdoorHippie. It's not a money thing, it's a walk don't run thing. And my wife had to buy into it as it can be a solitary hobby, especially given that she's an artist who shelved painting to bring up the children. I'm hoping the creativity in the house will re-ignite her creative juices - that appears to be slowly happening.