Well, that may be weird, but I think the reason why Gates to inf was so bad is... XY. Those games had to represent a revolution and created such excitement that Gates to inf didn't have to eclipse them, especially with the poor after game that XY proposed. You could say "why did they produce them, then ?" : because they have to keep a certain rythm.
Doing a game without much passion isn't Chunsoft, for me. They didn't have much choice but they didn't give us a totaly empty game though, it has good points :
- the weather ! How much I wanted to see the weather scroll (oups, right term ?) continuously since la Forêt Givrée (sorry, don't have the English name) ! ;)
- those doors that hide entire corridors, great idea.
- dungeon graphics that are just super.
They included those good points and some others to show us that they still can do great things. Because hey, they know that include all pokémons is one of the most important things they have to do, they aren't dumb... Thats why I am sure that this Mega mystery dungeon (or super where you are), will be great : there isn't the expectation XY generated ;) (sorry for my English, I am French :p)