How to disable mouse acceleration in Yosemite?
It turns out Yosemite enables mouse acceleration by default. it can be disabled by typing this in terminal:

defaults write .GlobalPreferences com.apple.mouse.scaling -1


After changing this setting in Terminal, you'll need to logout and log back in to your Mac.  Relaunching Finder will work too (option + right-click on Finder icon in dock -> Relaunch)