I think you will find, if you look more closely at Google results, that there are as many in favour, as against, but that is a personal observation. the adverse comments, though, imho, tend to be very much of a sameness and focussed on only a couple of new features which were not welcomed. There is also comment about "kernel changes". This can also be ignored. Any alteration to such a large program, however small, would require kernel changes.
I have found that 8.1 is slightly more responsive, but this seems to be a much varied opinion on the web.
Booting directly to the desktop is now an option. This boots you right to desktop, bypassing the Metro screen.
A cosmetic change that appears to be welcom by many users, is the ability to make your Metro background match your desktop background
Search has been improved. Instead of searching only your computer, it will now includes internet searches as well..
The Metro screenis now more customisable. You can drag tiles around the screen and resize them through the option bar at the bottom. But, as a built in safeguard, you can't accidentally starte any apps from this screen, whilst customising. The tiles can now be shrunk into three altenative sizes. One drawback, (for me) is that, in the very convenient smallest size, there is no text, so you have to remembr what each pretty picture does.
You can now see and change the display resolution and mouse settings from inside the Metro settings app, not a big deal for me, as I only have reason to perform such customisation in my initial setup. There are no options to turn off the hot corners. App updates can be put on automatic, instead of waiting for permission when you enter them, or the store.All popup notifications can be individually turned off-
Windows 8.1's desktop start button, on a left click, launches you right to your Metro screen. This could previously be done, already, with the Charms bar and the Windows key, so not a gigantic leap. A right click, however, does offer some more options, including shutdown/reset, command prompt , Control Panel etc.
One change that will appeal is that Metro apps are no longer restricted to a 30:70 split when you're using two at once. You can now make it a 50:50, or a 60:40. Unfortunately, the downside here is that some apps have not been fully amended and cannot make full use of the split options. Nevertheless, it is a big and welcome advancement for those who need to multitask.
The most controversial, and overhyped, item was the start menu, almost universally referred to, by the complainers on the web, as the start "button". When the beta testing began, without prompting by others, I saw the "modern" screen as my new program start menu. Thus the windows key, one click, and now, the re-established start button, fits perfectly for me. A left click and I have my customised array of shortcuts. A right click of the button shows several items which I have previously, for convenience, had on my legacy desktop - Control Panel, shutdown and restart options, Command prompts etc.
On the whole, 8.1 does move towards some improvements, but mainly to appease those who wished it to be more like earlier OSs.
But, referring to your original post, there are minor improvements, possibly to your personal advantage and it is free. So the real question, I suppose, should be, why not upgrade?