1 They find the graphics bad
Many people hate Minecraft because of its graphics. They believe that the game's graphics are bad.
2 They don't understand why it's popular
Some people hate Minecraft because they don't find it interesting and they don't understand why it is popular.
3 They find it everywhere
Because Minecraft is so popular, its videos and pictures can be seen everywhere on the internet. Some people hate the fact that it's that popular.
4 They find it boring
Many people hate Minecraft because they think it's boring.
5 They don't like its fans
Some people hate Minecraft because its fans exaggerate in praising it.
Some people, especially game developers, hate Minecraft for its tremendous popularity. (See The Body Language of Jealousy)
7 Gameplay can be different than watching
Some people hated Minecraft when they saw videos of it or when they saw others playing it. They may have hated it because the Gameplay is very different than watching the game being played.
8 It's not free
Minecraft is not a free game. From its earliest days it was sold for money. Some people don't like the idea of buying a game.
9 Some people find it pointless
Some people find the game pointless as it doesn't have a plot nor certain objectives, and because it doesn't end. (See Why are endless running games so popular?)
10 Some versions have bugs
Some people have reported some bugs in some Minecraft versions. They don't like the game because of that.