There's nothing particularly superb about the mystery of After the Thin Man, nor does it contain any innovative filmmaking. The screenplay creates some nice moments and setups, but does not break any new ground where dialogue or narrative are concerned. It is the sheer charisma and skill of William Powell - and his foil Myrna Loy - that makes the Thin Man series a treasure. I go into these movies knowing the ending and a few of the punchlines, and yet Powell's Nick Charles is such an entertaining cad that I still delight in following his exploits for almost two hours.
I suppose that it's rather shallow to simply love an entire series for the strength of its lead character, but I'm not sure that I've ever found a character that's quite like Nick Charles. He can be kind, deadpan, silly, witty, manipulative, accepting, populist, blueblood, and drunk. Some of Charles's characterization can be attributed to the screenplay (married scribes Albert Hackett and Frances Goodrich were no slouches), but the burden ultimately rests upon Powell and he pulls it off swimmingly. His double takes and stares make for great comedy. His sharp delivery in investigations creates wonderful suspense. His and Loy's ability to play off each other is fantastic. Most importantly, Powell can pull off wittiness without any trace of being self-righteous or grating. There have been charismatic stars before and after William Powell, but to me, no character has had the same magical appeal as his Nick Charles.