How does ubuntu render fonts better, than other main linux distributions?
Solution 1:
There is a bit of subjectivity (to say the least) on fonts and font rendering.
See : http://blog.mageprojects.com/2011/05/27/linux-fonts-compared-ubuntu-11-04-linuxmint-11-fedora-15-windows-7-sabayon-5-5/
That first link has a screen shot of several fonts on several OS and the author concludes :
The Ubuntu and LinuxMint fonts look ugly and way to fat. period!
(Authors words, not mine).
There are a few resources you can look into, you will have to decide for yourself what you like best.
A few resources you might look at (you mentioned you were using Fedora)
https://wiki.ubuntu.com/Fonts#Font_settings
http://blog.andreas-haerter.com/2011/07/18/tune-improve-fedora-fonts-typeface-ubuntu-like-sharp-fonts
http://fedorasolved.org/Members/khaytsus/improve-fonts
Rather the telling you how to configure you fonts, I hope those links help you understand how to configure them to your personal preference.
Solution 2:
I don't think that Ubuntu have any specific changes to font rendering that are not available upstream. Ubuntu does use it own font (called Ubuntu) which you may like more than fonts used by other distros. You can check if setting other distro font to Ubuntu font makes them look the same.
Solution 3:
One reason may be that there are software patents covering certain font rendering techniques. US-based distributions (such as Red Hat) have more difficulty distributing these techniques than EU-based distributions (such as Ubuntu).