Getting used to font rendering/smoothing in OS X

Yes, I've noticed it too, but as you've stated - only on an external monitor.

It might has something to do with a low pixel density...

It's rough on a 1920x1200 24'' HP monitor, but it is fine on a rMBP (obviously) and also on a TBD.

 

OS X and Windows use different strategies for sub-pixel rendering:

http://www.smashingmagazine.com/2012/04/24/a-closer-look-at-font-rendering/

Windows hints the font to fit in the pixel grid, this means slightly sharper edges at different font sizes at the expense of fidelity to glyph shape:

http://www.codinghorror.com/blog/2007/06/font-rendering-respecting-the-pixel-grid.html

When I moved from Windows, to be honest my first impression was that I much preferred the OS X font rendering. Although I program, I'm also a typography geek and hated Windows sacrifice of the glyph on the pixel grid alter. I did have to find a good font and font size for programming, however writing in a word processor was not an issue as changing view results in much better legibility IMO (the font design is actually used, not mangled). This is a subjective preference, and lots of Windows users hate OS X font rendering. EDIT: on the other hand enough windows users love OS X rendering that there are several projects completely replacing cleartype, for e.g. http://code.google.com/p/gdipp/

Note that as pixel density increases, the OS X method becomes clearly superior. Glyph rendering on OS X on the rMBP is amazing, you can see very subtle differences in the font design, and it is buttery smooth to read.

 

At "standard" PPI, I definitely prefer Windows font rendering. Much easier on the eyes. At sufficiently high PPI, OSX looks great, though I don't have enough experience with Windows at high resolutions to draw any conclusions there.