Your score is your opinion, which of course you are entitled to. Bottom line though, you didn't even finish the game's main mode.
The only situation where this might be acceptable to me, in a review, was if the game was so bad you couldn't bring yourself to finish it, but this should correspond with a score much lower than 7.8.
As is, it's an incomplete review so you can't blame people for calling you on it.
Having completed NSMBU and NSLU completely i would give it an 8.5, but i could totally understand and respect a score of 7.5 from someone who had actualy seen all that the game had to offer.