Ubuntu user here and very occasional Windows user too. Is the problem only with the GUI (same as liking or not Gnome Shell or having it crash) or is the problem with the "real" OS under the GUI?
Example 1 for the GUI: it drives me crazy that I can't resize the dialogs to edit the properties of a scheduled task. They were probably designed for 800x600 screens and they were a bad design back then (a text area please and join the lines.)
Example 2 for the core: a process keeps track of its parent but if the patent exits the process doesn't update the reference so you can end up with a reused process id in the child process data table. I run into that a couple of weeks ago.
I don't relly develop for Windows, so most of my problems are related to OS GUIs and bundled software. Like how for some reason, most of the bundled UWP programs stopped working on my laptop after an update or how Windows 10 still supports two audio "channels" (default and communication) which many apps respect but the output device switcher only switches the default one and not the communication one and these options aren't even available in the "new" (it's been like 5 years at this point!!) settings app so you have to use the old "control panel" one which has now been removed from the menu and sometimes even search and the only way to get to it is to search for something related ans when it opens switch the tab to what you need even though this is not some obscure feature but something that every mainstream communication program uses and most users eventually run into and everyone constantly bitches about yet there is still no fix or even acknowledgement from MSFT because they fired all their testers/QC and care less about their paying customers than even the meanest of OSS maintainers.
</rant>
Regarding the more technical side, while I'm entirely unqualified to comment on kernels and low-level APIs, I actually like some NT/Windows design choices quite a lot more than Linux. Network transparency (\\shares) is a big one and in general the way external filesystems are handled (mounting makes no sense for desktop use - especially the way udisks or whatever does it). And while the way programs are stored and installed on Linux is a neat idea in theory, it all breaks the moment one app doesn't follow the standard and it imposes far too many restrictions that Windows doesn't suffer from.
Edit: sorry for the rant, I'm stupidly tired and have had a few drinks. Happy holidays!
Example 1 for the GUI: it drives me crazy that I can't resize the dialogs to edit the properties of a scheduled task. They were probably designed for 800x600 screens and they were a bad design back then (a text area please and join the lines.)
Example 2 for the core: a process keeps track of its parent but if the patent exits the process doesn't update the reference so you can end up with a reused process id in the child process data table. I run into that a couple of weeks ago.