Good intentions up the wazoo, and I do appreciate the performances of Tracy, Hepburn, Poitier, etc., but this just isn't a great film. It's didactic as all hell: characters literally just speak the themes and messages out loud on screen. There's no work for the audience to do, which I think is what generates the negative criticism of it being patronizing. I woudn't call it "bad," exactly, but it doesn't need to go on the rocket ship, especially since In the Heat of the Night is from the same year and deals with the racism of the era in a more interesting/entertaining way (I'm not even a huge fan of that film either, but I'd easily vote for it over Guess Who's Coming to Dinner).
And this isn't about light touch or happy endings being devalued. You can have a light touch but still let the audience do some work and come to their own conclusions about the material. The Birdcage was discussed in the episode, and I'd say while that film (and its French predecessor) also has a light touch, it does lead the audience to a conclusion of acceptance/tolerance through character and situation, not by preaching at them. Guess Who's Coming preaches.