Up until very recently, Apple and Microsoft had very different approaches to rendering fonts. Because of Apple's presence in the publishing industry, Mac OS X focusses on maintaining the original shapes of the glyphs of typefaces, even if that means additional blurriness. Microsoft's ClearType, on the other hand, optimises glyphs for on-screen readability, forcing them into the pixel grid. This distorts some of the shapes.
Since I'm reading on a computer screen, I prefer the ClearType rendering method. However, it usually only takes a few minutes to get used to Mac OS X's shape accurate rendering, and it doesn't actually bother me. In the end, it's the consistency that matters - not the actual method used. Mix the two on the same display, and one of the two methods will start to annoy you (depending on your preference).
In Windows 8's Metro interface, fonts were fuzzy, while on the desktop, they're razor ClearType sharp as ever. I assumed this meant Microsoft had switched rendering engines (or tweaked the ClearType algorithms) to produce accurate shapes, readying itself for displays with high DPIs. In addition, although I'm not sure, I think shape-accurate rendering is a better fit for the kind of smooth zooming you see on tablets.
As Long Zheng found out, though, ClearType as a whole isn't missing - only its most defining feature is not available: subpixel RGB optimisation. This technique uses the subpixels in LCD displays to improve glyph rendering, but by using only the red, green, and blue subpixels, this causes the edges of characters to display tiny coloured fringes. Some people find these coloured fringes incredibly annoying.
Without this technique, ClearType is reduced to more basic grayscale rendering, which produces - in my view - sub-optimal rendering. However, unlike what I initially assumed, this is not the same as the kind of shape-accurate rendering Apple employs.
There are several possible reasons for this (the fact that tablets can be rotated easily comes to mind, making it hard for ClearType to adjust to the different subpixel orientation), but I'm sad to see ClearType take a few steps back like this. Font rendering on Metro suffers for it.