Yeah I just finished it myself and I agree. The game is fun to play for sure, but there's not much going on in it.
The main story missions are kind of un-eventful. They don't have anything special or unique to them. In really good GTA missions you'll have something-a situation, senerio, set piece something-that makes that mission special. No story mission in infamous ss has that, they all kind of follow the same routine. It dosen't help that the story structure they built makes that obvious. You basically do the same pattern of missions three times for the three important story characters(kind of anyway since the third guy you don't do the same missions you did with the other two). So the main story dosen't have anything standout and then you're faced with the world where you do the same thing over and over again as you attack the bases, find the shards, and whatnot. I mean sure playing it's fun, but there is'nt anything built around to elevate the entire thing.
Also the powers are kind of iffy. There's little deviations in what they do. The neon grenade makes you shoot them to subdue them while the smoke grenade allows you to just subdue them. The neon laser has slowdown and you have to shoot the feet to subdue, the smoke allows you to shoot in the head to subdue. Yet at the end of the day they kind of all do the same thing and it's up to what you like the most I guess. This game seems to not like Neon powers since it hardly has any part where it makes a point of you using them.
I also felt like the game kind of just abruptly got to the end, but then maybe thats good. It did'nt overstay it's welcome.
The thing though is the story is pretty bad. It's bland, predictable, and most importantly nothing happens. It moves in a straight line with nothing of interests really to shake yp the story. The villain is incredibly terrible and is just I'm a villain until the end where the game decides to just info dump her motives all of a sudden. It's just a really uninteresting narrative in the end. I like Delsen. He's a cool character and a lot of the dialogue between characters is funny, well done, and pretty natural. It's just the actual plot is pretty ho-hum.
It's a good game and an early PS4 game so it deserves some slack. It's pretty fun, make no mistake. It probably just won't be remembered when the better infamous comes a long.