In a user interface, similar things should look the same. But different things should look different.
Why did over 75% of Facebook's users think that the May 2009 Facebook UI redesign was bad? Because it made different things look similar to each other. Nobody could tell if they were updating their status or writing on somebody else's wall, because even though the text was slightly different in the box depending on what you were doing, the box itself looked the same. Similarly, the new Chat UI (introduced a few days later) made idle users look basically identical to active users, except for a tiny icon difference. (It's also important that different things are different enough, not just a little different, because people often won't notice little differences.)
This is an easy pitfall for developers to fall into because developers love consistency.
Everything should be based on a single framework, in the backend of an application. But that...