The only major miscast is Leslie Howard as Ashley Wilkes. Howard is way too old for the part, and although the Ashley in the book is a milquetoast, he's also a charming, dignified, lovable milquetoast. Howard drains all color from the character, and frankly is dull as dirt every time he's onscreen.
Howard isn't miscast; it was logical of Selznick to think of him. His type has fallen so utterly from fashion that it's easy to forget he was a major romantic star in his day, causing quite a few hearts to throb. He looks much older in GWTW's Technicolor than he does in Intermezzo
opposite Ingrid Bergman around the same time, and just a year earlier he was a wonderful and very attractive Higgins in Pygmalion
, so good Rex Harrison was reluctant to take the part later because he thought Howard's performance definitive. I'm sure the GWTW Blu-Ray does him no favors, either, nor any of the other actors who went through that stressful shoot.
I'd also say that Ashley is not a milquetoast. Mitchell notes that he can ride, shoot, and gamble as well as any of the other young men. He just doesn't care to. He is weak, but many men would look weak stuck between Scarlett and Melanie; there's that terrible scene in both versions where these two strong women descend on him at once to dissuade him from starting a new life in New York, and Ashley can't bring himself to lay down the law. I guess that's a wimp, but I'm not sure he'd be any more attractive if he went against his wife's will and dragged her North, unless you imagine yourself in a marriage where your husband orders you around and takes no account of your wishes.
As for lovable Rhett - bear in mind he tries on two occasions to lure Scarlett into a life of sin -- once by proposition, once by seduction. He tries to ruin her, in other words, before he gets to a proposal. As he tells her in the book, he's a bad influence. He may love her, but he's a dangerous guy.