This movie, even though it does get pretty good ratings, I still think it's underrated. The best performance--and one that I think was truly Oscar-worthy--was that of Jason Robards. He holds such gravitas that his presence--both for his humorous turns and for his serious turns in this movie are what anchored this movie. Same goes for Diane Wiest--who I believe was nominated--she was outstanding in this. And kudos to Steve Martin for being "the star" of the movie but stepping back where necessary and allowing these other actors to shine.
This movie is miles ahead of the related TV series Parenthood, which Ron Howard was also involved with. The TV series was utter useless dreck in this reviewer's opinion--it was too heavy, too depressing, and the "comedy" bits seemed forced, were not funny, and felt out of place. While this movie, in contrast, was more comedy-centric, with some drama thrown in for balance, and here Howard achieved the perfect mix.
So plot-wise it's four sub-stories of Martin and his character's siblings--Martin, his two sisters (Wiest and Harly Jane Kozak), and his wayward younger brother (Tom Hulce). They all have some sort of life issues, and the movie is centered around a holiday (or birthday) get together and takes turns following each couple as they deal with their respective issues.
Martin's character has a son who may have some sort of ADHD or learning issue--or maybe he's just a normal kid going through normal childhood bumps in the road. This is affecting Martin's job and to some extent his marriage. The sister played by Kozak and her husband (Moranis) are over-achieving yuppies who insist on the same "success" for their 3-year-old daughter and are prepping her at that too-young age. Hulce, the brother, is failing one get-rich-quick scheme after another, and is only visiting this get-together to borrow more money for the next scheme. Finally--and perhaps the best sub-plot, is Wiest's issues. Her 12-ish son (played by a very young Joaquin Phoenix) is going through a tough time because the father left Weist as a single parent and doesn't want anything to do with any of the family. Wiest's daughter, (Plimpton) is in her late teens and is dating a well meaning but currently nowhere-bound guy (Keanu Reeves). Most of the best humor revolves around this story. Wiest and Plimpton are at each others throats due to both general teen angst and the fact that it's tough being a single parent.
So there you have it--all of the plotlines keep the pace going and keep the viewer's interest high, and they keep the tears of laughter coming as well as the occasional tear of emotion from the drama. Very well done movie.