I like Sinnoh too but I understand why lots of people are against Unova.
The layout of Unova is just so plain, basically a big circle.
In Sinnoh, you were given a more complex map and it included a bit of backtracking and ways to get places with multiple routes. In Unova however, it was one straight path going from one town to the next.
From what I have gathered, people don't like Sinnoh because the don't enjoy the story. I've chatted with many people and this seems to be the general opinion. I don't see anything wrong with it myself though.