The episode itself is very groundbreaking and full of impact without any forced sentimental pathos (they are all natural, in my not so humble opinion...). It explains also the genetic smartness of Lisa too, although some aspects were somewhat exaggerated. Homer was portrayed differently than other episodes and he was not out of character and the writers did a great job on the progress of his aggressive side in the 1st act and the opposite in the latter acts. In my opinion, everyone was in the character. Mr. Burns turned to his dark side again after showing his warmer side in 'King Size Homer' (I wouldn't really deduct points for Burns remembering Mona, after all, he's special
)... heck even Marge had her moments. The plot was great, not special-great, but it made sense with some dramatic materials and the turnaround featuring Chief Wiggum in the very end was pretty clever. Humor was great--not particularly Season 5-esque laugh-out loud type--but good enough to make some big deal out of. Overall, the episode is witty, charming, emotional and serious with everything working well till the ending.
A+