One of the aspects that mar the MGM Marx movies for me is the revamping of the brothers' motivation. From Night at the Opera on they played the "good guys," ready to jump in and give the distressed young lovers a helpful leg up. And so you have Chico using his skills as a con artist as a power for good, filching Groucho out of his hard-earned in Day at the Races only to hand the ill-gotten gains over to Allan Jones to further his prospects with Maureen O' Sullivan. Or Groucho laying a reassuring hand on Diana Lewis' shoulder in Go West and saying, tenderly, "Don't worry...we also serve as best men...at weddings." <shudder>
The Stooges could get away with this kind of thing without it jarring too badly, since it's in keeping with their "well-meaning dimwit" personas. But having the Marx Brothers playing the angels in the post-Paramount films takes the characters pretty far from their origins as anarchic instigators of mayhem for its own sake. (OK, they were a LITTLE sympathetic to the lovers' cause in Cocoanuts, but the romantic plot was pretty tangential in that one). I think they're at their best playing inhuman forces of nature, so humanizing them for the MGM pictures seemed to take some of the bite out of their routine.
Despite all that, still plenty of great stuff scattered around in those MGM movies (um, except for Big Store). I had the same reaction to Go West when I first saw it - couldn't understand the hate. It's on the lower end of the Marx's quality curve, sure, but in its own right it's a serviceable little comedy. I remember watching the safe-cracking scene and wondering what the Marx fans were complaining about! 'Course it's been a while since I've seen it...I might hate it if I watch it tomorrow.
And At the Circus got a big laugh out of me with the cigar scene ("Funny, I don't have any trouble getting 'em!")