I am a heavy user of Mathematica, and I keep working with 12.0, because 12.1 is unusable for what I do:

1) my favourite keyboard shortcuts don't work any more in 12.1;

2) my collections of Manipulate panels behave erratically in 12.1;

3) with 12.1 I cannot export Graphics3D to vector pdf any more; I wonder if the PostScript engineer at Wolfram has retired and the position canceled.

I had been excited at the Presenter notebooks with scalable fonts. My impression is that the functionality has been abandoned half-done. To make a plot that scales in every detail is possible, but very time-consuming. I have not found a way to make the Tooltips and the Manipulate interface elements to scale with the screen.

I repeat, I love Mathematica, but I also have complaints.

It doesn't help me much. I'm thinking of buying a new computer (but the old one is still working great after 10 years!) Martijn's post makes me worry that a new computer will not completely eliminate the problem.

One of the problems with fonts is that different fonts can display with quite different sizes even if they have the same FontSize specification.

I wonder how many different fonts and font Styles are used by WRI in Mathematica? Their entire display quality might be more stable if they restricted themselves to a small set of fonts that displayed with a uniform size, with the same FontSize specification, and standardized their use over versions. If a user writes style sheets and uses other fonts then he's responsible.

There's no reason for WRI to use a different font just because they can. Writers shouldn't have to become system programmers and experts in type setting to make their notebooks look good.