Simply wonderful.
First off - the game is outstanding. This is just the sort of fun and fast game that will easily distract me for over an hour without me even realizing how much time has slipped by.
The animation quality of Sly is brilliant, and the controls are fluid. He moves the way you want him to move without any effort, and it all feels very natural. Kudos. The backgrounds, while a bit fuzzy, were really beautiful and it enhanced the mood and feel of the game without being distracting, which is perfect!
I loved the minimalist approach you took with the sound - the rain and thunder was soothing and unnerving at the same time. The fact that Sly makes no sound at all really makes you feel like he's quick and quiet - just like a real fox. The music, when playing, is mysterious and beautiful, and it really sets the mood without drawing attention to itself. I'm so glad the music doesn't just loop. It never gets old or repetitive.
The only issue I have is with the ending of the game itself. Given the general ease of play and "casual gamer" appeal that this has written all over it, I thought the ending was abruptly challenging. It wasn't too hard to beat, but it did seem like the game went from Easy to Very Hard in a matter of seconds, which takes me out of the flow of gameplay. It would be easy to fix, if you chose to - just make the game a little harder to beat once you're nearing the end.
Other than that, I thought this was really amazing. I loved the control - loved the animations - and I loved the sound. I can tell you put a huge amount of work into this. You should be proud.
Thanks for sharing this great game!