Just like the previous Wii System Update, 4.2 offers little in the way of surprises or features and mainly provides "behind-the-scene fixes," designed to "improve the overall system performance." The update also removes unauthorized save data (it wouldn't be a real update otherwise!), so if you've been using things like the Homebrew channel, expect to be annoyed for the few minutes it takes the community to implement a workaround.

Source Joystiq