Woke up randomly, couldn't go back to sleep, so I decided to play a few more levels.
So far and I'm still in World 4 I'd say the worst design decision of the game is the timer. I hadn't noticed it because I wasn't going through the levels too deeply, but I did notice it in a few recently. I don't understand why it's there. The game is fun enough on its own without the additional challenge, so why bother? The timer is kinda mitigated by checkpoints: if you die, you go back to a checkpoint and you have more than enough time to finish a level, but that's one life wasted.
The level design, what I really like about it, is that if it's not non-linear (as in, more than one path to get to the end goal), it's still offers a lot of choice through power-ups and little things to discover. Even though these levels are smaller, I feel like there's more to discover than what was in your typical Galaxy level.