James Franco ruined it for me
First of all I would never imagine Tristan looking like James Franco. He's doesnt have that british aristocratic knightly look, he just looks to american - i dont mean it as an insult but even physically he does not work as tristan.
Perhaps more importantly, he changed probably not more than two different facial expressions during the movie, neither of which was convincing. While I found Sophia Myles's performance excellent (and generally i think she's well cast) in their together scenes it's him who seemed blunt and fake: it's because of him that the chemistry between them didnt work. In terms of expressing emotions he was cheesy at best. Never could we read real torture, love, dilemma from his face.
He totally ruined it for me. I think the movie would've worked much much better if it wasnt for him, he totally killed the atmosphere, the feel of the movie. And it's a shame really, this movie could be good, had it gone just a little bit deeper, a little less cheesy, and of course, if it wasnt for franco.