Interesting question. I actually spent several minutes thinking about this, and I realized that I've mostly stopped tweaking per se. One thing I really love about Unity, is that additions doesn't plug in like in older versions. They just communicate with Unity, which in turn integrates it. I have added a weather indicator, for instance, which is really nice because it means I don't have to look outside my windows to see if it's raining or not. :)
I have added a few static quicklists to some of my apps, though. I have shortcuts for my terminal so I can attach to a remote screen very easily. I've added one to open Gedit with files relevant to a project, but that's not really useful and should be built into GEdit as dynamic quicklists instead.
But, while thinking about these things, one thing you might consider a tweak that I can't live without, became obvious. bzr. I've put my config directories under version control. This means that I can always see when a configuration has changed, exactly what has changed and I can always easily revert to an earlier version. That's quite nice and I'd recommend it to anyone.