The reason many people hate Windows 8 is due to it being a big change from 7 and other previous editions.
Yep. When I first heard of Windows XP and saw screenies and the changes that were being made, I hated it. I avoided it for years.
When I finally upgraded the windows 98SE machine I was using to XP (I forget why I decided to do so), I continued to dislike it.
The first thing I did was change everything to be as close to Windows 98SE as possible. I disabled all Luna themes, and I set the start menu to "Classic". One may strike a parallel to this with Windows 8 and people using aftermarket Start-Menu tools. Over time I came to discover that the newer start Menu actually had several benefits such as pinned applications, so I switched to that style. I also realized that despite my misgivings, painting a few bitmaps was not the "memory hog" that I originally chided it as, and switched to the XP Theme (Silver if memory serves, but eventually Royale Noir).
What changed with Windows XP from the time I originally used it to the time that I came to use it as my everyday OS? Absolutely nothing. The only thing that changed was me, and the only thing that changed me was cold hard facts, and I like to think that is the only thing that changes my mind about things now. Which is why I look at new Operating Systems and ways of doing things not with scorn, but with wonder. Windows Vista got the same treatment, since I had come to realize the "error of my ways" to turn a phrase. I bought a laptop with Vista preinstalled and had some issues with it, but I reinstalled Vista fresh and found that experience about a bajillion times more favourable, thus I learned that manufacturers were probably to blame for a lot of the complaints. I was thus quite quickly aware of how blatantly misinformed a lot of the anti-Vista posts and opinions really were. Then I watched a dramatic shift as those same people said how great Windows 7 was, and how it fixed everything wrong with it, when it hardly changed anything- it was just a re-release so that people could like Windows 7 and still hate Vista rather than admit they were wrong about Vista, because they are pretty much the same OS with a few tweaks- certainly nothing worth changing the minds of millions of people.
When I first learned of Windows 8, I had my concerns. However I also realize that the people creating teh software in question are not stupid. Despite what any idiot youtuber may try to say, they are not professional designers. I'm not a professional designer. But Microsoft has a lot of them. I considered that the changes were done for a good reason; rather than dismissing the changes as being done out of malice and responding in kind, I assumed they had very good usability reasons for the changes, and did my best to embrace those changes and merge them into my workflow.
Now I have no issue moving back and forth between Win7 and Win8. Eve nin Win7 I found the win8 UI style useful, as some of it carries over in Office 2013.
I think maybe some of the problem is that while most such changes require a bit of effort, relearning, etc. people expect the improvements to be for free; or, they expect simply incremental improvements to what was already present. The problem is that such an expectation only leads to a messy System that has a bunch of features and capabilities stapled on, while not actually addressing the usability concerns of previous versions.
This isn't to say that such attempts at improving usability are always good. For example, Office 2000 introduced "Smart menus", which had numerous UI oversights that Microsoft silently acknowledged by removing that feature (or at least disabling it by default) in later versions past Office XP.
In order to really understand the direction, you have to understand the problems with the Start Menu as implemented.
Now, as we all know, it was added in Windows 95. The design purpose was to make the components and installed programs easier to access while also centralizing a lot of thee system to a system-based menu. In this goal it succeeded. Detractors claimed that Program Manager did that just fine. Arguably, they weren't wrong, but the Taskbar was quite a novel introduction and made Windows computing easier, since running programs were always visible. It was like "Cool-Switch" (Alt+Tab) being available at all times. And the Start Menu was sort of like a "program manager flyout".
Naturally this paradigm pretty much stuck through to XP. XP reworked the start menu a bit to try to improve usability and of course add some glitz.
Vista is where we see the paradigm start to break down as computing changes. With massive hard drive we started to be capable of installing hundreds of applications. With XP and earlier having this many applications installed absolutely destroys the ease of use of the start menu, as the menu is forced to either create multiple columns, or scroll.
Thus Vista's Search was created. It was more or less an extension of available Desktop Extension tools (Windows Desktop Search, I believe) but built into the Start menu. IMO this is the single biggest usability improvement ever added to an OS. It found it's way into other systems like Linux Distributions too.
Very few people that use Windows 7 probably use the All Programs Menu. and if they do, they are going slower than they could be. Start Search is usually the way to go.
So where does win8 come in. well, it's a two-pronged approach to changing technology. First off, it's obvious that Touch-screens are coming around and going to be on desktops in the fairly near future, so it makes sense to start designing in a way that is compatible with that technology. Additionally, it addresses some obvious problems with the Start Menu architecture. Since it is most productive when used as Search, it is limiting that the results are confined to the menu itself. The Start Screen shows these results in full screen, allowing for far better utilization of the available space for both changing search options as well as organizing results.
That is really all there is to it. The Start Screen is a better Start Menu.