nkedel

Not applicable to upgrading, unless you have a very recent machine (e.g. a PC from the last year with Thunderbolt) or are in the habit of upgrading a desktop piece-by-piece, but the biggest reason is better support for new hardware, and support for newer hardware going forward.

The second biggest reason if you're on Windows 8 or 8.1 is that it's basically smoothed out the rough edges of those operating systems, and essentially every down side remaining is already true for either of those.

For folks on Windows 7 on older hardware and happy with it, there's no really compelling short-term reason to upgrade. There's a lot of nice incremental improvement, and a bunch of stuff that works just enough differently to make a power-user unhappy to have to re-learn things. OTOH, in the long term this is where Windows is going, and you can bite the bullet now or bite the bullet later.

For someone who doesn't have much invested in already understanding older version of Windows, I think they're likely to find it easier to use.

Lay-person incremental improvements:
1) Search feature works better.
2) The new "File history" snapshot + backup feature is really, really great, especially compared to having to use two different things for that.
3) For new users, the ribbon-ified explorer is probably easier to use. With 17+ years of muscle memory between Windows 95 and getting a Windows 8.1
4) New file copy and "there's a file with the same name" dialogs are much better.
5) It's more secure. Setting up multiple user logins is easier. If you're willing to trust Microsoft, the user-settings sync is nice. If you're willing to trust Microsoft, the OneDrive integration is nice.
Quite a few etc. None of these especially compelling, but for a free upgrade and a chance to catch up to the present day, nice enough.