One of the things, for me, was that Debian's releases were confusing. Ubuntu had a clear schedule, aligned to a calendar, with a clear out of life policy.
The second thing was that I couldn't figure out my way from debian.org, to the relevant ISO to download. There's just so many options, scattered explanations, I felt overwhelmed by the site and couldn't figure it out reliably.
The third thing is the out of box experience. Every major Debian descendant has this mentioned in the articles that compare them. Ubuntu is like a refined, user friendly Debian. Mint further polishes Ubuntu. Elementary is designed for newbies and to have excellent OOTB experience. And so on.
Fourth thing, that Debian Stable seems to be behind in version numbers, and Debian Testing seems to be not stable enough. Maybe these are not true, but they haven't inspired confidence in me either.
Fifth, Debian came with some proprietary things off by default, so no WIFI on my laptop, no MP3, bluetooth, other minor inconveniences. I love open source, but I'm not a purist, and I wanted to make use of my computer first, and improve on it later.
After Ubuntu pushing their current in-house bullshit again (snaps this time), and Mint behaving weird on my main computer for some reason (maybe the multiple release upgrades), I took the plunge into Debian after some research, and ended up on using Debian Testing with KDE. So far, the experience has been good, but this really feels like a Linux enthusiast's distro, where I had to look up things on the Internet, because the installed just haven't explained anything - me, with 10+ years of Linux experience, ending up with installing a system that won't boot into the graphical desktop, despite selecting the KDE option in the installer!
Debian is a pain! I just tried installing it on a laptop. During installation it points out it can't find a wifi driver and do I have it to hand? I didn't and I don't know where to find it. It says I can use ethernet, but the ethernet wasn't close to me at the time and laptop's battery was shot so I couldn't move. I proceed without internet. I google how to install non-free firmware after the fact, and find that there is an app in the repository. Once the install is done, I shut it down and move it to a room with ethernet. We boot up, plug it in and get internet! But for some reason I am not attached to any software repositories apart from those on an install CD/DVD. I don't know what has happened, but I assume that my lack of internet during install meant I don't have apt sources properly configured. I considered googling how to add these, but instead, I said goodbye to the early 2000s gnome 2 theme, shut it down and tried another debian-derived distro that has all the advantages of debian, a beautiful modern theme, and I don't spend hours googling things I don't know.
I use Ubuntu because it is relatively current at release, has a very regular schedule, and my preferred LTS releases have 5 years of updates.
I occasionally talk to a Debian user. Ask how they handle their long term Debian use. Which usually takes them several minutes to describe. XYZ from stable, ABC from unstable, track the latest with foo from testing, and bar from backports. Basically they end up with a custom setup, unlike any other on the planet, and that hasn't been tested.
LTS on the other hand does get testing, generally I just apt install the packages I need, and then do-release upgrade every 2 years. LTS is popular enough that generally if I google the problem there's a quick fix, and a large fraction of LTS users don't seem to customize their repos to the degree that seems common on Debian.
I don't think there's a single answer to that, but I got a few points that may be the more prominent ones, in no particular order:
a) Their (now changing) stance against unfree firmware files. This made the default adverdised installer fail most laptop installations due to the WIFI firmware not being available, so no internet to pull down the required packages to make it work. There were unofficial ISOs, but they were hard to find and so a frustrating experience if you didn't know about it. Same, but not as worse with other unfree stuff, like NVidia drivers, you are required to enable the non-free repos, which makes totally sense if you check it out more closely but still a bump in the road for users new(er) to Linux or only used Debian derivatives that often have a stance that is (relatively) less freedom oriented.
As hinted, the firmware situation recently changed and will get better with the next major release next year.
b) It's more universal and flexible to go in various direction, so it comes with a less opinionated out-of-the-box experience, for the better or worse (depending on your preferences). For example, Ubuntu provides a Server and a Desktop edition, and then all the semi-official Desktop spins for KDE, XFCE, ..., but Debian doesn't you can get it to move in either direction quite easily once you grasp its basics, but that naturally still is an overhead compared to just pulling a preconfigured "ready to fly" version ISO.
c) It is definitively is sturdier, but due to their policies guaranteeing that it's also less bleeding edge, well at least if you use the stable release. You can run the Debian Sid version (unstable), but the project doesn't want to promote that to inexperienced users. FWIW, I run that on my laptops since years and got an excellent experience. It's a rolling edge version of Debian, fresh as Arch, but I just align a bit better with Debian (plus develop on it at work).
There are surely some more, but those where the biggest three popping into my head.
The great thing about Debian, they got quite a healthy relationship to most of their derivatives. Which could IMO only happen due to not being run by a major company or the like, but 100% community based. While that sometimes makes for slow turning mills, but still (or maybe exactly because of that) an excellent and safe foundation for their users, be it direct ones or indirect through derivatives.
My info might be out of date, but I think for a long time Debian was not including some audio codecs like MP3 because there are some patents around it. I think there are some proprietary drivers like NVidia that are also excluded.
I think the Debian distro has always targeted/attracted free software purists.
I understand but nothing changes for me, I already used the non-free ISOs. The only difference is that they will put the non-free ISOs at the front and the regular in the back. The ISOs will be the same.
