I liked the way you had Snape changing back from his teenage self after the flask had broken, and that Harry needed to get to like him in his "old" form, rather than just continuing to like him as teenage!Snape.
The first Defence lesson was brilliant. The way Harry managed to bind Snape, but then was unable to say that he hated him - that was very well done.
One of the best things about the story to me was the way you captured Ron's dialogue. I could really hear movie!Ron saying all those things. You've written a very convincing Hermione as well. I liked all their scenes together a great deal. At the same time, I loved the way you had Harry being so hesitant, and tongue-tied, and struggling to cope with his growing feelings for Snape. I think you wrote all of that amazingly well.
I also loved the comparison between teenager Snape's and grown up Snape's kissing technique. Lucky Harry that he got to experience both!
Altogether a pleasure to read - I was sorry to get to the end, I wanted it to keep going! ♥