I don’t really know the lingo, I just know what I see.
Look at the image below. Can you tell which is which?

The left is XP, the right is Linux (Firefox, mstt core fonts, and mlind’s freetype/xft/cairo updates with David Turner’s newest lcd filtering patches, full, BCI-enabled hinting). I took the screenshot by overlaying a VMware window running Ubuntu on top of a Firefox window on the Windows host.
As far as I can tell, the problem isn’t actually in the subpixel filtering. It’s more that, in the Linux example, the “thickness” of the parts of each letter are inconsistent. For example, diagonal, and curved parts look much thicker than the rest of the character. Arial seems to exaggerate this effect. When there’s a bunch of small text clumped together like this, the letters just look fuzzy and dirty.