* Natural, responsive scrolling. After two-finger scrolling on OS X where it's so responsive that it's exactly like sliding paper over a table, nothing can really compare. Linux can't get to that point for a long time since it's limited to sending scroll wheel events.
* Palm and thumb detection that work like black magic. I have never once had palming issues on OS X or issues with resting my thumb on the pad and using other fingers to point, click and drag. I've spent days on end trying to tweak the Linux Synaptics drivers to work the same way with no luck.
I don't have a proof, but this doesn't seem to be the case for GTK3 apps. Scrolling in Evince (GNOME's PDF viewer) and Web is very responsive.
YMMV. I have had such issues.
In Chrome you can set the default zoom for pages, but you will have tiny tabs.
Until you connect a non-HiDPI external display, where everything will still be scaled up by a hilariously huge factor of 2.
This is exactly what I'm referring to as High DPI support. Maybe it's not the solution you'd prefer but it is a solution. By contrast, Linux still has a lot of fit-and-finish issues with high-DPI, even in Gnome 3. I would believe that they're going about it in a better way on the backend, but the fact is OS X is the only one really delivering at the moment.