Toolbars, menubars, etc

Screen space is precious. Don't waste it with clutter! You either reduce the area available for showing the user's data, or you cover other programs. Programs should use drag-and-drop to work together and small windows make this easier.
In general, you should start off with the most minimal interface possible (ie, just showing the data). Add extra user interface features only if they are needed. Do NOT start a new application by creating toolbars filled with non-functional buttons all around your window!
Toolbars:

Toolbars are generally OK if they're small. Don't put too many tools on the bar though -- searching for the tool you want may waste more time than just using the popup menu. Users are more likely to investigate five tools than thirty.

Menu-bar:

Try to avoid these. A menu bar is slower to access than a popup menu, and wastes screen space. Only use one if you have a very large main window and lots of menus.
The preferred method for giving commands in ROX applications is the popup menu. Popups have the following advantages over other means:

• They appear instantly, right under the cursor. No time wasted moving the mouse.
• They are arranged in a hierarchical structure. All the functions of the program are shown in one place.
• GTK+ makes it very easy to bind keys to menu items (by pressing the key while pointing at it).
• Popups don't waste any screen space when not in use.

All toolbar functions should also appear on the menu so that keys may be bound to them.

Status-bar, Preview panes, etc:

These are usually a bad idea, but it depends on the application. Put them in if they become necessary.

Wizards

A wizard takes the user through an operation step-by-step, because the operation is too difficult for the user to perform on their own. The need for a wizard usually indicates a problem with your user interface. It is often better to fix the problem than to write the wizard.
For example, consider the task of making a chart in a spreadsheet when the user clicks on the 'Chart' tool:
A wizard-based approach would bring up a long series of dialog boxes asking the user for the chart type (bar/line/pie/scatter), title, axis labels, grid lines and so on.
A non-wizard approach would display a chart with suitable defaults immediately and let the user change anything that was wrong using the normal editing tools.

The second approach has a number of advantages:

• It's usually faster, because the user only changes things the program got wrong.
• If the user is trying to make a sketch graph quickly, they don't have to mess around with things like the grid type which they don't care about.
• The user feels comfortable making changes to the graph later. A wizard-user will often have to delete the chart and start again every time they wish to change something.

It's often better to tell the user how to do something than to do it for them. A good example is setting the program to handle a particular type of file when no default has been set. ROX-Filer tells the user to use the `Set Run Action...' menu item; they'll know how to change it again later. Explorer brings up a dialog for them; most Windows users have no idea how to change a file's run action a second time!