I think that some sort of Linux based distrubution (like Ubuntu) would be a good choice for almost anyone. Personally, I haven't run Windows (except for as a testing environment) in over two years now.
There were a couple concerns in this thread that I'd like to take the time to address.
One, most Linux based operating systems tend to get improved quickly. To use Ubuntu (a likely choice for new users) as an example, their release cycle is every 6 months. While there usually aren't dramatic changes in the user interface, new features are added and broken ones get fixed. So, if you're someone who tried it out a year or two ago, and couldn't get it to work right and found that it wasn't for you... It may be worth taking the time to try again because the things that weren't working may now actually be fixed and working fine.
Two, getting help when you need it. Getting Linux related help can seem kinda daunting because online, sometimes documentation isn't written for the average level user, it's written for a developer level user... And it's not as easy as just bring your computer in to the local computer store often because many of those types of stores don't know any more than you do about Linux and then try to put Windows back on the machine to "fix" it. I personally recommend getting hooked up with the user groups in your local area because they're a great resource for help, whether that be them helping you learn how to do things yourself or someone you can hire to help. (http://www.linux.org/groups/
has a list of LUGs and for Ubuntu users, https://wiki.ubuntu.com/LoCoTeamList
- getting hooked up with your area LoCo team can be a resource).
Three, command line. Because of the way history has gone down, Linux based operating systems seem to have an automatic association in most peoples mind with the command line. Here's a little secret... You don't need to know anything about the command line to use most Linux based operating systems on your desktop or laptop.
Most anything you'd need to do can be done through the user interface. It's just that some people really like using the command line to do things and they forget that other people don't when they give instructions.