I've namechanged for this as I don't want to out myself.
I'm from one of the other Scottish Islands. Similar population. I lived on the mainland (city) for years and have now been back for a while, having longed to come back for years. Have primary school aged kids. My situation is slightly different I guess as we have family here and this place is as much a part of me as my family. It's in my blood. I truly do mean that. I feel like it's part of my DNA. Much as I like where I lived before I know I would never have felt that it was home.
In terms of practicalities - fuel is expensive but you won't ever be stuck in traffic so we actually spend less time in the car over all.
Food - Tesco's may be taking over the world but their prices are the same in all stores.
Property - depends where you live now but I would be very surprised if you find it to be anything but cheaper / better value. Council tax here is low - not sure about Orkney.
Internet shopping has come on in leaps and bound in recent years. There are some that charge extra or won't deliver here, there are many that don't. I use Next, ELC, John Lewis, M&S, Debenhams, Tesco and Asda regularly. I only use online shops that offer free or cheap delivery and don't find it a problem.
Activities - Things have moved on so much since I was a kid. Having been to Orkney I don't think it will be any different - swimming, gymnastics, horse riding, judo, karate, dancing, music lessons, football all kids activities that are readily available. Not to mention space to run about, beaches etc...
Fresh air and beautiful surroundings.
Very little crime - my car sits in my driveway with the keys in the ignition all the time.
For nights out there are pubs and restaurants.
I would advise anyone to get involved in local activities. Gala / fun days, ceilidhs etc. All take organisation. Help is always appreciated and it's a great way to get to know people.
Respect locals and their way of life and traditions and you'll receive a warm welcome.
Oh, and their is no such thing as bad weather - only inappropriate clothing 