Ah, I think I know why Opera is messing up fonts now -- it seems to be using FreeSerif for Arabic, for example (I was wondering just now why Opera couldn't shape a Person cheh), which doesn't seem quite up to scratch for the task. So, let's pick a different font for Arabic and Hebrew rather than going with the default.
Edit: And after I did that, all the Latin text changed shape, too.
Fortunately, I knew that I had been able to fix this before, and I must have written an entry about it -- and Google found this entry. And indeed, setting the Hangul font to "Automatic" fixed that problem.
Opera, you're bizarre.
Addendum, 2009-02-13: Greek Extended, this time.
Addendum, 2009-02-14: Halfwidth and Fullwidth Forms. Really, Opera, what's going to be next? Do I have to change everything to "Default"?
Addendum 2, 2009-02-14: Runic? Hebrew? This is getting more and more bizarre.
Addendum, 2009-02-15: and now you're taking my Japanese sans-serif fonts away from me, too? I prefer "gothic" fonts to Ming-type ones. Grrr. This may just be the last straw. Don't make me do this.