It's using a custom font called "ET Book," which is (according to its GitHub) inspired by Bembo.

If I had to guess at what you find grating, it might not be the letter shapes, but rather the spacing. At least rendered on a Mac, the inter-letter spacing just looks off; it's a little too tight all around, and certain pairs look inconsistent. It lacks visual smoothness. (I know that sounds like somebody talking about wine or oxygen-free audio cables, but I can't think of a better phrase.) It's possible that this is a quirk of Apple's font rendering algorithms, but I'm not convinced -- and it's definitely the font. If you prevent external fonts from loading or use the web inspector to remove "et-book" from the CSS and let it fall back to Palatino, the typesetting improves.