I'll admit that Google tends to do pretty well with usability, but I've got two beefs with the AdWords My Client Center interface:

1. The lack of a Campaign drop down menu
2. Inconsistent button labeling

The MCC has a drop down menu that lets me jump between client accounts. Why don't they have another drop down menu that allows me to jump between campaigns? If I'm looking at one campaign and then want to switch to view another, I have to go back to the Campaign Summary page and select again. Sure, you might say that a Campaign drop down menu would only save one click, but it sure would be more user-friendly (and every click I save has the potential to add up to a significant efficiency boost.)

Why is re-activating an Ad Group labeled "Resume", but re-activating an Ad Variation labeled "Unpause"? I'm pretty sure "unpause" is not a word–why doesn't Google just stick with one label and call the re-activating button "Resume" throughout the interface?

These two simple fixes would provide the MCC interface with a better score on usability.

UPDATE: Everything's better in three's, so here's another gripe about the AdWords MCC interface: When you generate a list of potential keywords with the keyword tool, there is a link that allows you to "add all" keywords to your list. I would like to see a link like this for the negative keywords as well. As it is, I have to click each individual negative keyword to add it to my list. What a waste of time! Come on Google, help us be more efficient and happier marketers!