Basically, Saki is about a small high school Majong club. It's members are from various years and are almost all female (one guy, but he's usually ignored anyway). The show includes a team tournament (most of the show), an individuals tournament (squished into a few episodes at the end), and random fillers inbetween (training camp, swimming, etc.) Using Majong is quite original, although the plot is otherwise ordinary.
This is one of those shows that has TONS of characters. At first, it's a bit confusing, but they did a great job of introducing them all during the team tournament. By the end of the tournament, I could place any of the characters in their school groups. In a few of the other anime I watched this season, they couldn't pull this off so the show turned into a confusing mess.
Character development was common and interesting. There was also the interesting and unexpected cute shoujo ai between two of the major characters, which was original. The only character that really bugged me was Nodoka, because of the size of her boobs (it's not an ecchi series, so they were mostly ignored, but looking at them all the time was annoying).
I have no idea how to play actual Majong. However, just like in Hikaru no Go, they make the game so interesting and entertaining, and the main standings easy to understand, so you always understand generally what's going on and it's fun to watch. The ending seemed a bit rushed, but it made me quite excited for the next season.