I found repartitioning the harddrive by far the biggest hurdle. That's a complicated and scary process that can delete all your data if you hit the wrong button. Picking the right partition sizes is another problem, as the Windows default EFI partition for example is far too small to be used with distributions that put their kernel on there (e.g. NixOS), but there is nothing warning you about that and resizing it, is complicated since there is a Windows partition in the way. The solution already existed in the form of Wubi, which made your whole Linux installation a file on your Windows partition, but that got sadly abandoned.

Next biggest problem is the boot manager, they still suck and are far to brittle. I'd wish we got rid of boot managers as is, and instead just booted into a mini-Linux has boot manager, that could not only be used to fix bad boot configuration, but also used as full recovery system. Having a full OS as boot manager means you can update and change the whole OS without fumbling with USB sticks and stuff, you can even update or switch distributions remotely. It's an extremely powerful setup, that as far as I know, none of the popular distributions uses.

Finally, just having stuff work. My amdgpu driver still crashes regularly. There is always some obscure crap I have to configure to make things work. And I regularly have to search the Internet to find solutions for my problems. Can we have some (opt-in) Telemetry here? A tool that can scan my hardware and error logs, tell me what I have and tell me if it works in Linux or direct me to an bug tracker with workarounds? ProtonDB for hardware, kind of. Why do I still have to do that manually?

Another big hurdle of course is just the software, even if everything runs perfectly on the Linux side, moving all your software over is always a big hurdle. Wine/Proton helps a lot, but still fiddle for stuff outside of Steam. Not really seeing any easy solution here. Something like Xen installed by default that lets you switch OSs without dual booting might work, or a VM that can boot into your actual Windows partition, but no idea if that would work well enough to solve more problems than it creates.

All that aside, the problems for new users are a bit overrated. Installing Linux is something you do once or twice, that process of course needs to work well enough to function, but it's far more important that the OS works well once you are past that point. If the OS fails in daily use, that's when people abandon it. Enduring a shitty installer for a weekend is not really that big of a deal in the bigger picture, if the OS you'll end up with is actually worth it.

Little aside: Why the f' is 'parted' not the command line version of 'gparted'? As far as I know, there is no command line tool left that allows you to move and resize partitions via command line in a single UI. That functionality was ripped out of 'parted' years ago, so you are stuck with manually fdisk, ext2resize, etc. which is not fun at all, since they all take sizes in different units and have different UI.