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It all depends a lot on how one uses Windows. For browsing and free webmail service there isn't much that one needs. If one can find the Internet Explorer icon then that's enough. If one doesn't mind being spied on by Microsoft, Google and a dozen advertising data researchers then Win7, Win8, whatever... it doesn't much matter. Win10 might be even better. It's locked down more and with its own AV, so security will be less of a concern.
For people who install a lot of software and like to control their system, there are a great many differences between versions. XP installs at about
1 GB. A disk image can fit on a CD. There are very few system restrictions. But there is Product Activation. With earlier versions of Windows one actually owned a copy of the software on disk and could install it at will. (It was still licensed for a single machine at one time, but one didn't need to get permission from Microsoft to install it.)
With Win7, a basic install takes up 7-9 GB and grows from there. Access restrictions are extreme. Under normal circumstances there is no such thing as a user with status to control the system. Microsoft is the Administrator. Microsoft claims the right to have the computer call home. Again, all of that doesn't much matter for people who have no idea what the file system is, who don't know where their files are, and who don't care about privacy. People who live by MS Word, Picasa, dropbox and various ad-supported services may never even notice that they're locked out of the Windows system. But for "power users" it's a pain in the neck. And there are numerous less noticeable issues: Since MS started refusing unsigned drivers it can be extremely difficult to get hardware installed in some cases. In the future look for more restrictions on software. Already, starting with Vista, software has been getting gradually pushed out of access to the system. In some ways that's good, but it's all part of a gradual trend toward locking you out.
Each version of Windows has generally been a further step toward locking out access and turning the product into a services device. With Win10 Microsoft has control over what the system will be. Not only will they install updates without asking (some updates cause big problems for some people) but they may also decide to change the interface, install software you don't want, etc. It's Windows services now. Expect ads. (By default there are already ads on the Start Menu, masquerading as "suggestions".) There are also indications that MS may start charging monthly fees once they've got a large number of people settled in Win10:
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None of those thing are necessarily bad for everyone. People who shop online and use freebie services may find later versions of Windows more user-friendly. That's the lesson that Apple learned: The more restricted the system is, the more stable it will appear to be and the more usable to the general public. But if you like to open the hood, so to speak, then the progression of Windows over the past few years reveals a discouraging and insidious trend.