Bad luck and poor decisions have hurt you this year. I don't think you can blame one single thing for your poorness this year as it has been a combination which have stunted you.
The Summer - Poor window which you failed to address problems which you've had before Moyes began at United. A new signing or two would have injected life into the side and could have improved your First XI more. Also, the one signing you did make hasn't put in the performance he's capable of but i could forgive him for that given that he isn't fully fit and is injured right now.
Injuries - You always seem to have an injury list but it's hurting the players you need most. Rafael is a big loss given that you don't have a replacement RB, RVP is the best in the league when on form and Rooney is a must have this season it seems. Add into that people like Carrick who is your best CM and it's going to be really tough for Moyes anyway.
Moyes - He has made some poor decisions at times but that is part of the experience. Martinez will make them with us at some stage, i'm sure of it, every manager will. He's under immense pressure so it's not surprising and it's a question of whether he will learn to cope with that pressure which hopefully for you he will.
Other teams - I don't think people would have envisioned how good Arsenal, Liverpool, Us, Newcastle, Southampton (who have gone a little off the boil) etc., would have been this year and this has made it a little tougher for you given the combination of other reasons above.
Players - Ferdinand, Evra etc are getting older and you can see they won't be able to start for much longer, but Moyes had no clue that they'd suddenly drop so much. Overall the performance of the team at times hasn't been good enough. There have been team-sheets which everyone has been pleased with on here and the players just haven't put in a performance expected of them.
That's my take as a neutral anyway.