I think we all expected this to be mediocre; but I personally have different reasons for doing so. It's not the "New" series aspect, although that bleeds into it somewhat. I actually really like the New Super Mario Bros. games, especially NSMB2 which was a really clever little game under the admittedly bland aesthetic. It's a shame not many people gave it a chance, because NSMB2 was probably one of the best games in an otherwise uneventful 2012, but I can totally understand what turned people off from it. And there's no doubt that some of the NSMB series' worse habits have gone straight into YNI.
But honestly, I knew YNI was going to be mediocre because it just retreads all the same tracks of YI, which if you ask me is also a mediocre game. YI had a great aesthetic which was made to look even better for its time by the SuperFX2 chip's graphical capabilities, but it's sort of the converse of the NSMB games in that an aesthetic that was anything but bland was wrapped around an absolutely brainless platformer with annoying mechanics to boot (don't tell me anyone liked the Baby Mario mechanic.) The actual platforming was piss-easy, the levels simplistic, and the powerups you got weren't things that actually made the gameplay more interesting as much as they were diversions. Maybe it's one of those games that indeed loses some appeal when you get older and - unfortunately - it's one of those rare Mario games I didn't get to actually play until years later, but YI was anything but the masterpiece people often claim it is. It was all flair and no substance. Kirby's Epic Yarn did what Yoshi's Island tried to do but executed it much better in terms of the game actually being interesting and having depth, and it's because of that game why I have higher hopes for Yarn Yoshi than I had for this.
In any case, I often argue against claims that Nintendo just rehashes things over and over, but YNI fits that bill perfectly. No one asked for this game, and I think everyone saw scores like this coming. Though personally, I could tell this was gonna be dull from the words "Yoshi's Island" in the title, not the word "New."