The thing is that there has always been evolution with these characters. People are upset they aren't getting smooth operator TAS Lex. Okay. But that's not the only one, now is it? Lex started out as a criminal mastermind (sometimes with fangs... Da hell!?), that morphed into a renegade scientist which his appearances in the serials continued, Havkman's Lex was a full of himself schemer out for wealth, then Byrne turned him into a heavy set tycoon captain of industry, then he was a freakin' clone of himself that masqueraded as his own son as John Shea was the TV version of Lex which was at first a full head of hair poor man's take on Byrne's Luthor. TAS Lex was a refined version of the post Crisis version and Smallville gave us the intensity of a Lex almost destined by parentage to become a cold blooded villain as portrayed by Rosenbaum. Spacey seemed a more menacing modernized version of Havkman's jaunty but arrogant villain (which was what you could expect I suppose given the nature of that film), and Morrison went back to the well of the renegade genius sociopath obsessed with taking down a Titan from another planet.
What I am getting at is that, yeah, I understand that Lex is a character that has expectations due to his well established history but let's remember it's also a history with a rich and varied portrayal across media. Taking the long view indicates that the character has been malleable and can be done in ways that are not simply in the mold of the most recent popular versions of either Rosenbaum or Brown. I especially think that judging it before seeing it is jumping the gun. I don't assume tha Lex is just going to be as he is in that party sequence all the time since there is ample evidence in his other appearances and lines that point to something else.