GameTrailers gave it a 4.5... I don't think I've ever seen a MP game score below a 6 from major review sites.

You are taking the lowest score from the 50 reviews...
66 on meta is normal for this game, MP 8 was 62 (8 million sold) and MP7 was 64, so...

Loved it

Eurogamer gave MP8 a 4 and there are plenty of other sub 60 scores throughout the series...the series tends to average 60-75 on meta. Its never been particularly high scoring.

And yet, Eurogamer Italy says that MP10 is the best in the franchise...

As always with Mario Party, the games are devisive, and those who look at the series with a critical eye often find fault, however, those who go in with friends for a good time, get just that. Making comments expecting reviews for such a series to really mean much of anything is laughable as reviews are arguably least relevant when it comes to party games...

"Mario Party 10 brings family and friends together, which is the most important element of the franchise. It'll make you smile and yell, cry and cheer. It's built to pull a range of emotions out of you. Most of all, it's meant to create long-lasting memories with those you love. I know I'll certainly never forget the weekend of Mario Party 10 that I just had. Our group of friends were playing until nearly 2 in the morning, doing our best to steal just one more victory from each other. We had such a fantastic time, and Mario Party is the only franchise I know of that brings people together in this way." - Kevin Cassidy (GoNintendo)