Yeah, I totally understand your complaint. Take the following as me explaining why it (and many similar things) are done this way, and definitely not dismissing what you're saying:

Fonts are tricky.

It's hard to appreciate how tricky they are, until you try and make a website that's readable for everyone, everywhere.

Rendering bugs, for example. You get a font that looks great on a Mac—it looks terrible on Windows. You find one that looks good on Windows, it glitches out on Linux.

Then there's things that aren't "bugs" but more like "differences"—rendered font weight is different on Mac vs. Windows/Linux.

There's browser differences: Chrome and Firefox (or, more precisely, Webkit/Blink and Gecko) anti-alias letterforms differently (at least on Windows).

There's device differences: what looks good on a hi-DPI screen doesn't look so great on a lower-res screen (and vice-versa!).

Of course these things interact. (Otherwise it would be too easy, right?) Chrome on a Retina Macbook vs. Firefox on the same Macbook vs. Firefox on a Linux box vs. Safari on an iPhone vs. Safari on an iMac vs. Opera on the same iMac vs. Opera on Windows vs. Chrome on Windows vs. etc. etc. etc.—all are ever so slightly different in how they will render the very same text.

Sometimes you can correct for this. Pick a font that works best on most platforms; detect the remaining platforms, serve a different font to those. (GreaterWrong does this for the UI font—Mac and Linux users get one font, Windows users get a slightly different font. I am betting 99% of people won't ever notice or realize this… which is the point, of course; if we didn't take this additional step, then people would notice, because there'd be visible glitches.)

Often, though, you can't. (For example, there's a particular longstanding browser bug in how text renders on mobile devices (specifically having to do with hyphenation) that we can't properly fix, because there's no way to reliably detect whether any given client can or can't do that particular thing correctly. This is why the text column is left-aligned on mobile clients. The alternative would be having most users see well-formatted text, but having a large minority of users see very glitchy, hard-to-read text; this is unacceptable.)

And, as you say, some things are just subjective (at least in part; it's almost never totally subjective).

So while I absolutely try to ensure that everyone gets as close to a perfect reading experience as possible… compromises are inevitable (and I really do mean "inevitable", not "meh, too much work"; no amount of effort short of "literally be Apple or Google, and maybe not even then" would fully solve this problem).

All that having been said, though, could you tell me what sort of device/OS/browser/etc. you're viewing the site on? Every piece of user feedback helps! :)

Edited to add: Whatever corrections you make, even if we don't/can't adopt them into the "official" version of the theme, I encourage you to submit them to userstyles.org—it's likely that some other people will share your preference, and this'll make the browsing experience better for at least some of them.