This post is about the newest version of Apple’s desktop operating system, Yosemite. It’s a short post, because I’m only focusing on one aspect of it: the typography.
In short: Helvetica is a horrible choice for a system-wide, screen font.
I work on a 15-inch Retina MacBook Pro and I connect to a 27-inch, non-Retina display. My comments are equally applicable to both screens.
So what’s the problem with Helvetica as an operating system font?
Helvetica reads poorly at small sizes.
As it does in Finder windows. There’s too much letterspacing, but even if letters were tracked tighter together, it would still be less readable than Apple’s previous system font, Lucida Grande.
Helvetica was designed in 1957.
Yep, 1957. It was designed as a display face, and when it’s used as a display face at large sizes, like on a lock screen of your phone or better yet in signage, it can be beautiful.
I’m sure there have been modifications to Helvetica to make is work better on screen, but that’s like upgrading a ’57 Chevy with satellite navigation, new suspension and a new engine. Sure it’s going to ride better than the original, but a brand new, 2014, entry level Toyota Corolla will still drive better.
I came across a post by Eric Karajaluoto, he has a different opinion:

“What about this change to Helvetica?” you ask. It ties to the only significant point in yesterday’s iMac announcement: Retina displays. Just take a look at Helvetica on any high-fidelity screen, and you see a crisp, economical, and adaptable type system.

Sure, Helvetica looks crummy on your standard resolution screen. But, the people at Apple are OK with this temporary trade-off. You’re living in Apple’s past, and, in time, you’ll move forward. When you do, you’ll find a system that works as intended: because Apple skates to where the puck is going to be.
Deciding to use Helvetica is not designing for the future. I’ve seen the future on my retina display and still looks like shit.
Apple could easily spend a tiny fraction of their billions and commission a custom typeface that works twenty times better than Helvetica.
It’s a missed opportunity.
[Imagine if Apple evolved Helvetica the same was Porsche evolved the 911: modernize the face while maintaining it’s essence, it’s Helvetica-ness]