I'm not one to throw my toys out of the pram at these weird add-on softwares, DRM etc but it is quite annoying:
-Having to remember which account I should be using after reinstalling as it has somehow merged with other Microsoft stuff like Hotmail, Xbox etc.
-Having to wait for it to sign in when starting a game, sometimes it won't let me use some of the menu options until it has logged me into GFWL
-Popups randomly appearing me telling me I've earned 10 Gold or whatever, ruining the immersion (maybe I can disable this)
-I found the menu pretty cumbersome to use
-I seem to remember having some weird login issue that took ages to resolve a while back, had to completely uninstall and reinstall everything
-Unnecessary bloat as I never use any of this third party **** like friends, achievements, overlays, screenshots etc (note this isn't specific to GFWL, same goes for Steam, Origin etc)
-Some games force you to patch the game through GFWL, kinda annoying if I think I've got it fully up to date via Steam or whatever but then have to update through GFWL as well.