It's not just the smooth controls, cute graphics and excellent music (I like the change of tunes when you hit upon a "secret" level) that set this game apart, it's the quality of the level design. Very sneaky, but with a good gradient for the learning curve, and a difficulty level that can be challenging but rarely feels unfair.
Occasionally it's necessary to explore a level for a few lives to work out what's going on, but on the whole most of your deaths feel like it's "your" fault for doing something silly, rather than the game pulling a cruel trick on you. There are no hidden traps on booby-trapped squares, for instance, which is the kind of thing that really irritates me. The traps are blatantly obvious. How to time yourself to get around them might not be!
The collision detection can be quite generous - some parts of your body can survive being touched by a spike for instance. This sounds advantageous to the player but in many ways can work against you - for example, there are some places in between two spinning traps where you can safely sit, the spikes just brushing/safely overlapping your character, while you wait for the right time to jump the second trap. Judging purely by collision avoidance, you might not believe these safe spots are there... on the other hand, once you start looking for such spots, hoping for the game's generosity to come into play, a lot of other tight spots turn out to be (only just) fatal. That's frustrating. I'd almost rather that a tiny brush with a spike proved fatal to remove all doubt!