My feeling is that in a lot of post out here, people want Ubuntu Mate be something else than Ubuntu Mate ...
Look what @vkareh is mentioning 
My list is:
- Remove all ability to use themes
- Remove support for desktop icons
- Remove all accessibility features
and is not the only one out here
I think Mate should focus right now on three main things, used by user to interact visually with desktop.
- background wallpaper - resizing, HDPI ... as I understand that is not very consistent, and right processing algorithm.
- mate panels - a lot of improvement has done, but along with this improvement has appear some small bugs (I also see some wired text in new custom panel config, are new features not finished, or ...). So it look like this panels are in work, and it require more work to bring them to "stable", to finish them.
- Mate indicators - I don't like that small size of them on 20px panel with - because of some padding from panel border, witch other icons does not have. Also missing date near the clock. This look like it require more work, as jut replaced up and down and are not displayed well as in previous releases.
All this three elements you can see on all desktop all day long, by all people, and give the first impression when you load up Mate Desktop, even if some are background technical staffs, it affect the most important three elements of first Mate interaction.
Par example, I see in Github a lot of work on Caja, but personally I don't remember having an issue with it, wile right now the indicators are my permanent issue, as look worse than ever before.
So the conclusion, one issue that I mention is already on @vkareh list, and also on his list the OpenGL Marco staff is really nice nice feature, but is a future not bugfix, so on my opinion should have less priority than bug fixing, and finishing in work staffs.
But on the other side I understand that he can work on desktop on whatever he likes, because he is working on it from his passion, so I appreciate his work a lot.