Some of the newer tablets and also Notebooks (notably the iPad, Macbook Pro and the Nexus 10 tablet) use displays with ultra-high resolutions at around 300 dpi. This makes text look very, very good, but it also means that images that are not adapted to that resolution can look rather bad in comparison.
Here are some crops from a screenshot from my Nexus 10:
These show that the StackExchange button and the logo, which contain text and hard edges, looks noticeably softer on a high-resolution display. This is of course no big deal, but it is very noticeable.
It would be nice if at some point some of the common images on SE sites that contain text or other hard edges would be served as high-resolution versions to devices with such high-resolution displays. This is obviously not a critical issue, but it would be nice if it could be tackled at some point.