Biggest adjustment for me was the widescreen aspect of it.
Since the days of 800x600/1024x768/1280x1024, I was used to run most of my applications fullscreen...
In widescreen it often doesn't make sense to do that (with a browser or mail client for example, what's the point? Generally they are oriented as portrait-layout pages, so it's more about the height than the width)...
So I started using more and more applications in windowed mode, with more or less the same 4:3 dimensions as before. This allows me to use the extra space on the sides of the screen for IM programs and such, or for undocking certain windows and having them float to the side of the main application etc.
I'd say it's pretty cool, once you're used to it.