Lion exhibits the first signs of resolution independence, which has been something of a 'Holy Grail' of Mac OS X for most of a decade.
In Preview we have the option both for pictures and PDFs to either chose one pixel being a screen pixel or 100% being resolution independent.
Finally we got back to be able to measure sizes on screen, as a throwback to the times of 72 dpi-screens.
[crarko adds: The actual preference setting reads 'Size on screen equals size on printout.' I only see this in Preview but it's a good step to putting it in the OS as a whole.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110725023942258