why the fonts in word 2013 are blurry?

The Problem is Office 2013 itself.  The font-rendering system inside Office Apps bypasses every OS-setting and therefore this issue is unfixable.

Also it falls back to grayscale ( blurry or fuzzy , renders only black pixels , look depends on the scaling )  font rendering instead of RGB/BGR Cleartype ( always readable because it renders every Pixel ) which Microsoft uses on Windows Vista/7/8*.

Also DirectWrite is used fo Office 2013 ( and Metro in 8/.1) while Cleartype RGB/BGR positions fonts "free" while DWrite positions fonts via the GraphicsCard ( think of 3D Aero Rendering for fonts ), depending on the speed and capabilities of the GPU and its driver this can lead to "lags" while typing, as every single font needs to be calculated on "how to position" it on screen.

The Hardware Positioning in Office 2013 can be circumvented via Words/Excels/etc Options. But the grayscaling renderer can not be avoided, so definitive there will always be a noticeable difference from ealierer Office Experiences.

As Microsoft is releasing constantly Gigabytes worth on updates for this messy App Collection I have hopes it might get better over time, at least it started to not look *that bad* any longer on my machines compared to the versions of one year ago.

*And one word to Windows 8/.1 :

The very same font-rendering is used on the "metro"-side of Windows 8/.1 , this looks as bad as Office 2013. On the Desktop-side this problem is absent, because it uses the font-rendering used under Vista/7 - so as long as User skip and avoid "Modern Apps" , they wont notice it. For Office Needs I recommend Office 2010 under Win8/.1 which will look perfect beccause it obeys Desktop Cleartype Settings; while  version 2013does not as outlined above .

Modern Apps/ "Metro" renders in grayscale, Desktop in RGB/BGR

I did exactly as you suggested a while back and reverted to Office 2010 and it all looks fine on Windows 8.1.

It is also worth noting that IE11 behaves the same way as Office 2013. Both desktop and metro versions of Internet Explorer 11 ignore the OS settings and display dreadful blurry grayscale rendered text.

That is why Firefox is my default browser.

Microsoft's stubborn intransigence in relation to this fundamental usability issue is a spectacular demonstration of how to alienate users and lose customers.

Here with IE11(except error-fail javascript in forum of Google, open only in compatibility mode), the characters is normal here. But the MS Word 2013 use inverted subpixels and nothind do to fix the problem, I also went back to Office 2010.

Thank you for suggestion. It does not help.

I came here because I searched what is about my fonts. I have both 2003 and 2013 office isntalled. I went to open some text in 2003 just because I realized something is not ok. 

Than I opened same document in each program and put them side by side on two identical IPS displays, with same (150%) magnification, as I normally have on my 23'' lcd. I already switched off subpixel positioning, but that changed little. Word 2003 was better looking and offer less eye fatique, than 2013. Period. 

Than I used Snipping tool and make a Snip of both texts. A looked closely than how pixels are aliased in both programs. in 2003 aliasing is made with COLOR pixels around black font, so from distance text looks sharper and lighter. But in 2013 aliasing is made only with grey pixels around black font. What looks as bolder and less sharp text looking from distance. 

Yet another thing in 2013 office is not great. 

Thanks to this post, I know there is nothing to do, so I will stop loosing time searching for a hack and using my 2003 word for most thing, regardles I have Office 2013 installed too.