...the most important thing that I learned in Psych 110, the idea that when people are successful at controlling their environment they become happier, and when they can't control their environment, they get grumpy.
How do you achieve delivering a feeling of control to a user in an application? Well, good feedback and speedy response are critical elements, otherwise the user doesn't emotionally connect her action to the resulting behavior of the software. Oh, and predictable outcome from user action, of course.
Also, drat him, he got one of my hobby horse design ideas that I never understood why we didn't do at Adobe (I know that's ungrammatical)... A toggle to reveal to you the keyboard shortcuts for each app buttons so all you have to do is look and then type it. No annoying mouseover in order to find out and then force one to remember it. So simple, so obvious, so NOT DONE ANYWHERE (that I've needed it, anyway).
Brett also snuck in a feature he's been itching for: lots and lots and lots of keyboard shortcuts. There's only one keyboard shortcut you have to memorize, though: Ctrl+; switches FogBugz into keyboard mode and little letters light up reminding you what the shortcuts are for various commands around the screen. It's really pretty cool to be able to work through a bunch of cases, assigning, editing, and reprioritizing, without ever reaching for the mouse.