Oh, wow. This was stunning. One of the most brilliant fics I've ever read. With one of the most in character Severuses I've ever come across. My god, he started out as such a sadistic bastard, but that's so him! So canon! All of his prejudices and blindnesses with regards to Harry coming into play. It hurt so much how he treated Harry, how he deliberately hit all of Harry's weak points or treated him like shit, and was proud of what he was doing! Totally canon bastard Severus, and I just wanted to hit him over the head - a desire canon Severus and Harry both often inspire in me. Wonderful charaterisation. Have I mentioned that before? ;)
I found Severus's jealousy and possessiveness - way before he came to truly care for Harry - fascinating. After all, it wasn't some kind of instinctual reaction. He'd never had an intimate relationship with Harry that he was, on some level, protective of. Though I think those emotions are part of the connection that always existed between Severus and Harry. And I loved how this fic shows that, although Severus despised him, Harry has been the most important thing in his life.
The moment Severus realised that Harry is the most important thing in any universe? A moment of pure (meta) happiness for this humble reader. Because it's canon! Looking at the events from Severus's point of view, he lived over half of his life for Harry.
And your Severus kind of realised that. With his task over, after he had given everything to Harry he had to give him, he was happy to die. I think that was the moment I fell in love with your story. :) No, not story. Epic. *hearts*
OMG, the worldbuilding! The theory of parellel universes, with just enough explanations thrown in to seem credible (quantum mechanics, how does I love thee! ^__^). Original characters that were rounded characters; and that little tidbit about how "haning out" with fifth years had changed Paracelsus, OMG, past character development and a little portrait lore thrown in together. And all the potions knowledge details just blew my mind.
Not to forget the wonderful end to the relationship development. I loved how Harry went from "my Severus" to "the other Severus", how he learnt to differentiate between the two versions and to accept the present Severus as the same-but-different man. Loved how they travelled together and how it brought them closer. Wasn't that part of how Harry and the other Severus got together, too? And Severus wearing his hair in a ponytail because he liked Harry's reaction to it! I haven't forgotten that "hair in a ponytail" was one of Harry's fifty reasons. Guh.
Okay, shutting up now. I'll go and stand in that corner over there and quietly fangirl some more. :)