A few of you have misinterpreted this one - which is a pity; it's a good post.
You're not telling her she looks different to other girls. You're saying "it's been a while since I last saw you; you don't have the same effect on me you used to ... you know, back when I used to worship the ground you walked on", but not in as many words.
It takes her off her pedestal, yes, but the main purpose is to draw a big thick black line under your past AFC behaviour towards her, and move on.
You really want to communicate, "hey, you used to have me under your thumb, but I'm not so easily impressed nowadays, honey", but of course coming right out and saying that - or worse, throwing some banal insult at her - is desperate ... and transparently so.
"You look different" isn't an insult, at least not an overt one. But the girl can't help but get to thinking, "if I used to be a goddess to this guy, if he found everything about me perfect or as good as ... well, any change would be a step backwards ... right?"
"I almost didn't recongise you" serves to twist the knife that little bit harder. Very clever, very cruel. I love it.
To borrow from another poster, "maybe the way you carry yourself" would be a killer follow-up if she tries to pin you on specifics. Not sure I could keep a straight face through that one, but if you can, more power to you.
I am
The Main Event.