Most people have complained about either Microsoft downloading the operating system in the background (which would've been critical in slow internet connections), upgrading them to Windows 10 without their consent, or the constant pop-up messages that come from the Get Windows 10 application. Some even went as far as taking legal action because of this.
And then there's the usual privacy concerns and the whole "Microsoft spies on you" thing... but think about it, aren't most products like that nowadays?
But I agree. Upgrading to Windows 10 from Windows 8.1 (in my case) was pretty much a seamless experience. Nothing major happened other than having to re-adjust the settings. If people don't like it, then they have the option to go back to the older version of Windows they've been using before.
If the Start button (or perhaps the taskbar as a whole) doesn't work, you should try restarting the File Explorer (explorer.exe) process by using the Task Manager. If you still can't left-click it, you can always right-click the Start button to activate the Quick Link menu, which is pretty basic but gives you most of the options the normal Start menu would give you.
Have you found a way to re-install all your pre-installed applications? Checking out a bunch of tutorials (most of which rely on PowerShell) might help.
Thanks to my downgrade from version 1511 (the November update), two of the pre-installed applications no longer work: Photos and Alarms & Clock. I tried to re-install them using PowerShell to no avail. I even went as far as trying to get rid of them altogether. But I noticed that those were carried over from my 1511 installation. Perhaps if I go back to the newer version, they might work again.
Also, as far as I know, every time you update to a newer version of Windows 10, most of the settings reset to default, meaning you'd have to manually set Firefox as your default browser. You can check
this tutorial.