I've recently upgraded OS X to 10.9 Mavericks, and since this upgrade Emacs has problems with displaying unicode subscript characters (such as ₁, ₂). There are a few strange things about this behaviour:
it doesn't seem to depend on the font, as I've tried selecting several different Unicode fonts;
it doesn't seem to affect any other unicode character, as I've tried several uncommon mathematical operators, which display just fine. Even superscripts (¹,²) are displayed as normal;
it doesn't affect anything outside of Emacs, as the characters are displayed by my browser just fine.
Does anyone have a clue what could be causing this behaviour?
I just tried this in the version of Emacs you are using, 24.3, and it appears to be broken. However, I also built Emacs from a recent snapshot of the Emacs trunk, which appears to be working correctly.
You can download a prebuilt version from www.emacsforosx.com under Other versions -> Nightlies, alternatively you could wait for the next Emacs release which typically comes out around this time every year.