I do not like Monobook; I feel as if it wastes too much space on the screen, difficult to read, and doesn't follow established web usability guidelines. Some of NullBook's features:

• Fixed-position "tab bar", set to use GUI colors, that stays with you as you scroll down a page. The tab bar contains the contents of the "Views" menu, as well as the go/search box.
• No font size definitions; uses your browser's default. Skin (mostly) scales well with font size changes.
• Underlined links, and more recognizable link colors. Blue for unvisited links, purple for visited links, and green for broken (or uncreated) links. Moving over a link highlights it to red.
• Removed sidebar to stop wasting vertical screen real-estate, and relocated it to the bottom of the page. Also hid the other languages list since I tend to only look at English Wikipedia articles.

You can find more information about NullBook (including screenshots) by looking at the NullBook section of WikiMedia's Gallery of user styles.