I don't know if the level of judgement is worse in the WC than say La Liga or Premiuer League overall over the same mount of games to conclude a proper average. You got to consider that most of the games are rather high stake too where the teams are rather even and both are playing for the win. In typical leagues you got plenty of matches that are plain boring or between top and crap teams, making the matches less intense and way less controversial to be referee in.
Overall there's been some insanely big mistakes however. You also have to take into consideration that while in many cases the bad calls haven't affected the result for the winning team, every decision more or less has a butterfly effect. For example the Italy v Costa Rica match. If Costa Rica had gotten the penalty the actual goal likely wouldn't have come because the situation of the play would be different. Overall the number turned into "1" which was the justified number, in just a couple minutes later, but you can't be sure how it would've affected the play. However a wrong call is a wrong call, and you should always hope for the "invisible" ref that makes all the good calls to make the game turn out like it should. Overall I've only been impressed by the ref team in a few games. The worst mistakes have been related to off-side and penalty situations, and they are clearly the worst as they are goal related. The first Brazil penalty was so damn cheap, and the Spain penalty. But then a 100% obvious penalty for Costa Rica was ignored. It's really bad in those cases.