Sometimes when I defeat the boss (like the Aragog or Ashkore) the game does not register it, so I have to do it over and over again. I have to kill the Dark Boss to continue the story, but I’ve already killed him twice and got nothing from it, no achievement, no EXP, no nothing.
You need to deal most damage by yourself or with pets. If mob get most of his damage from other source like falling damage, fire or from other npc it will not register.
There isn’t much better than a wide open game world in which you have essentially no limits and can alter the very landscape in which you play – that is, unless you add dragons, of course…