I have experienced this terrible feature in so many sites (including Google’s Gmail): A drop-down menu that executes when you select an item from the list (as opposed to select and click the ‘Go’ button).
How does it hamper usability of a website? Read on:
1) It is so easy to select the wrong choice. If there is a ‘Go’ button, you can always retract and correct your choice.
2) On some sites, when you select an item, the respective page loads but the window focus is still in the drop-down. If you use the scroll-wheel on your mouse, it scrolls the item list in the drop-down, changes your choice, and leaves you with, ‘WTF did I do just now?’

Please have the ‘Go’ button. Please.