I can't understand why it got such poor reviews! Don't get me wrong it's not a perfect game by any means, but it is a pretty solid launch title. I'm only about 3-4 hours in, but I probably would have scored it in the 7.5 - 8.0 range based on what I've played so far.
There are only a few problems, so I'll address those first...
The dodge mechanic does not feel good or control well. It is usually easier for me to jump to dodge attacks than it is for me to actually dodge. That said, most attacks can be dodged by jumping... a few enemies will require you to use the dodge move.
As a platformer, I wish there was more platforming and exploration to the game. The levels do seem pretty well paced so far, but I do like platforming and puzzle solving in these type of games. Knack is lacking in this area. There are a few simple environmental obstacles but that's it. Some environmental puzzles would have been nice.
Although the growing in size mechanic does actually work well in the game, I can't help but feel it would have been cooler if the levels didn't dictate how big or small I could get. I would have rather grown large as I collected more health, and grow small as I lost it. This one is a more minor complaint though, because their system works well within the context of the game. When you get the chance to grow huge, it feels special.
The last minor thing... in these type of games, I expect to unlock special moves as the game progresses. In Knack, you have all 3 from the beginning. Unlockable moves would have added some depth to the gameplay.
Some of the things I loved...
I love finding and unlocking the hidden chests! One of my complaints was the lack of exploration, but finding these makes up for it just a bit. Part of the fun in finding them is having other friends that have the game! The rewards for the chests are randomized parts that help you build a gadget that will enhance Knack. You can compare your random reward with what your friends got from the same chest. If what they got will help you build a gadget you are going for, you can take what they got instead. It's a very simple, yet cool idea for social integration in a game. I hope we see it in future games.
The platforming is in the combat... although there is not as much platforming as traditional games, I love the timing/strategy required to beat groups of enemies. I'm playing on Hard. It has kept the gameplay fun and challenging, but not punishing.
So far the pacing is very satisfying. You never have to stay small Knack for too long, and the combat feels different at various stages of Knack's growth. More importantly, the combat is still satisfying whether he is small or HUGE.
Lastly, the story and characters seem charming. I kind of wish Knack's voice changed as he grew... and I haven't got deep enough into the game to see how the story unfolds yet... but so far I'm enjoying the characters and the light-hearted story surrounding the game.
It's not a perfect game, but it's not nearly as bad as some of the reviews made it out to be. It's worth noting that my affection for the game could be related to the fact that I love Japanese anime... and coming from the Sony Japan Studio, it does have a Japanese kind of feel-good quirkiness to it.