Mathematica, on the other hand, is squarely in the terrain usually covered by open source.
Mathematica, on the other hand, is squarely in the terrain usually covered by open source.
This is a matter of debate, depending on what circles you're traveling in. If you're working in a "mixed environment" where some people are using OpenOffice and some people are using Microsoft Word, you're likely to run into problems that range from minor annoyance to show-stoppers very quickly. Some of this can be blamed on Microsoft's weird file formats, but this is true not only for .doc files but for the better-documented (if arcane) .docx and, in my experience, even with RTF. I'm in contact with a fair number of small-to-medium press fiction editors, and while a fiction manuscript is one of the simplest use cases you can imagine in terms of formatting, almost every editor I've talked with has complained about OpenOffice (and to a lesser degree LibreOffice) screwing up comments and revision tracking. I don't really want to use Microsoft Word, but so far I haven't found an open source equivalent for my needs that is good enough. (The closed source Nisus Writer Pro and even Apple Pages seem to do better, ironically. But that's not to say they don't have their own problems.)
I tend to think of open source's most well-established terrain as languages and server-side components where UI is not really a major issue.
The fact that 90% of the world is using Microsoft's format is certainly relevant for everyday use, but it's orthogonal to the question of whether an open source project can achieve feature parity with a closed source product.
File format compatibility is always hard and I'm not convinced MS does a better job reading odt than LibreOffice does with OOXML. But LibreOffice does have a reasonably comparable feature set, even if the UI is not as attractive.
Pile up all the features Office has but LibreOffice doesn't on one side, pile up all the features that LibreOffice has but Office doesn't on the other. What pile's larger?
It's kind of a pointless argument over whether or not LibreOffice has ENOUGH features; enough features for who? It's a global question with local answers. It's pretty easy to count who has MORE features, though, and Microsoft Office wins there.
But, FWIW, most of the new "features" Microsoft adds to Word seem to be UI improvements (e.g. https://support.office.com/en-us/article/What-s-new-in-Word-...). And if you want to claim that Office is more usable than LibreOffice, and that this is generally true of closed source vs. open source projects, you won't get any arguments from me.
Still, for a lot of people, GIMP is a very valuable open source alternative to Photoshop; not as full-featured, of course, but then again, nothing is. There are lots of tutorials for achieving interesting effects using GIMP. I've never heard it described as something people learn to pad their resumes.
Krita is pretty cool, too. If I'm not mistaken, it's not a full-featured Photoshop replacement either.
And GIMP doesn't really lack many features Photoshop has, and even has some that Photoshop doesn't. UI could be much better (especially controls for transformations), but it's not that horrible either. The reason why GIMP sucks in comparison with Photoshop is that few features it lacks are absolutely game-changing. Like effect masks. If you use effect masks (and pretty much every proficient Photoshop user does), software without them is not an option for you. And I believe it isn't that easy to introduce them to GIMP either.
Like colorspaces, effect masks, etc.
I found Krita to be a far better photoshop replacement than GIMP, actually. (With two differences: moving selections is a bit more complicated, the option for that is hidden by default, and it does not have a simple contrast-brightness slider filter)