I think that's where Vaughn originally had the Inception-style room spinning mind battle sequence. Vaughn cut it because he didn't want to copy.
I think he didn't come up with an alternative version because he wanted to give all the cool action scenes to Magneto.
When Vaughn talked about that epic battle scene inside the mirrors' room, I was pretty sure that both Xavier and Emma would be part of it, even though in the end it wasn't that epic anyway. But there you go.
So, although I have my own petty nitpicks like everybody else here, the main thing I would change about First Class was turn it more into a X-Men story and less a Magneto - Origins story.
Reminds me of a comic strip I read a while ago:
"Wow, the new Magneto - Origins movie is awesome!"
"Ah, you mean the new X-Men movie?"
"The X-whaaat?"
I really appreciate FC for the movie it is - there's a good deal of emotion, an incredible score, nice action scenes, a touch of humour, cool cinematography, great acting, etc. But it most certainly isn't the film I supposed it would be, the one I personally hoped for. I really wanted it to be about two friends with different ideologies who decided to work together because they thought they could find middle ground, and in the end it was clear that they couldn't. So I wanted both characters to be flashed out equally, and not a film about a loner Bondneto set on revenge who happens to come across this enthusiastic fellow who reads minds and is very eager to help him.
I mean, what really are Xavier's motives for starting the school? Only to be the CIA's Mutant Division? How about his own ideology? How about how he mastered his powers? How about his own fears and doubts? James McAvoy is a great actor who was very much misused in FC, because really he wasn't given much to do other than to be a complement of Magneto's journey (a bit like his Yoda, perhaps?). McAvoy's wonderful enthusiasm and belief in the character he played gave Xavier its brightness and presence; a lesser actor would have a hard time making the character relevant, even memorable, like he did.
If they could've made this work out, and given the eletric chemistry between McAvoy and Fassbender, man, it would've been amazing.