You know, I could be wrong, but if you ask me, GTI felt kinda uninspired, I mean, it felt like the developers started enthusiastically but lost interest half-way.
1. There is only one game. While not a big thing, but having two versions for the spin-offs too was kinda nice. Also, we never got a "third", better version with Special Episodes and the kind. And I'm pretty sure we aren't getting it now either. GTI ended with one game.
2. Of course, the lack of Pokemon. I know doing all those 3D graphics is time consuming, but they could have at least given us 300 so that there was some variety. The world of GTI feels so small. Even while doing job requests, it feels like I'm helping the same Haxorus get out of Desolate Canyon 5th Floor for the 100th time.
3. There are just 5 starters. I know, I know, all your starters have special sequences where they use different animations like when they pump their fist, or get in a fighting stance before a boss fight, or go to sleep, or scream while falling off the skies, but the previous games had most of that too, but they still had at least 10 starters. The decision of only 5 starters felt less like "You guys should appreciate these dudes too" and more "There's no way in hell we're doing all these special animations for 15 critters" to me.
4. All the starting dungeons, you'll see, have cool field obstacles you must clear by reaching a certain floor and pushing logs off of it, or jumping on buttons to raise up stumps to jump to the next floor from. But as the game progresses, these start to disappear, like they were just there to mesmerize you during the first few levels so that you start playing. As the story progresses, you'll see that eventually these field obstacles are completely gone and it's normal dungeon crawling again. The obstacles were a very novel idea, but seemed to be ditched half way through.
5. There is almost no postgame. The only thing you've got it is that one time where you delve into the final story dungeon and bring your starter back to the Pokemon world. And... that's it. Game over. That's when the world starts to feel really lonely. Now you don't even have the story to accompany you. EoT/D/S did this so well, I expected an awesome postgame arc form this game too. But I was disappointed.
Now, I know I could get beat up real bad for saying this, but I'll risk it anyway - The game seems to be lazily done. It seems like it was done in a haste, actually. Like they thought we'd just buy it because of the glory of its predecessors. Its only strong points are the gripping story and awesome music. While this alone is a lot, you can't help but be disappointed because the predecessors had these two and other stuff to content you with. It would be a good game but the minute you compare it to the previous two, it sucks.