If you use Jetpack by WordPress.com, make sure you skip the update for 4.0. It had a fatal error that crashed plenty of sites, and was fixed with 4.0.2 (see announcement post).
This is a bummer, because it reduces trust in the auto-updates WordPress can do. The perception that even a plugin from Automattic can’t get it right doesn’t help. However, Jetpack is a weird meta-plugin, which goes against the grain of how we’d prefer plugins to work.