Really good feedback is extremely important. Even simple things like making it obvious what can and cannot be clicked can be overlooked or too subtle. Feedback when something might happen in the background is great. In gmail, it's great that there's a status ribbon appearing at the top that let's you know if something is sending or loading, but it's even better that it lets you know that something has sent successfully or is still loading.
The "yellow fade" technique is something else made popular amongst the RoR crowd that accomplishes something similar. You never want the user to ask the question, "What just happened?" or "What will happen when I do this?".
Another trick that has become more popular lately that I've been using a lot is editing in place. Instead of having a view of some data with a separate "edit" screen (or skipping the view and only having an edit screen), it can often be more user friendly to have a nicely laid out view of some data and just click to edit parts of it. This technique is really only appropriate when reading the data happens more often than editing, and is not appropriate for serious data-entry.
