By far 4.3.1. is my favorite Xcode release :) A lot of those tiny details that disturbed me are gone! Here is my short list.
1. When you create a new group, now you can edit its name in place.
2. Now when creating an objective-c class it creates a private intefrace for you. That is a cool thing for me as I was often too lazy to do it myself (shame, I know)
4. It marked now as warning all those places where I forgot to call [super dealloc]. My fault, but very nice from Xcode as it helped me to find few leaks I was hunting for some time now.
5. Installation experience. Installed from the appstore with no single issue, offered to remove older versions and had no single issue with existing projects. I was afraid of burning the night oil again, but not this time! Oh, thanks.
6. No crash since installing. I was used to Xcode crashing 2-3 times every evening. Great time saving again.
7. Last but not least! I've been using lldb for some time now, but was always running into "Invalid service" etc kind of problems when I even had to restart a device and Xcode from time to time. Since I've installed 4.3.1 NO SINGLE CASE when I had to do this! Saves me really an hour a week at least :).
I guess real list will be much longer. It is just what I ran into during the first few days.