After re-installing XQuartz (which is prompted for when you try to launch IDL), IDL seems to work fine. But I haven’t had to install IDL on Mavericks and that usually seems to be the worst problem.
The Xcode command line tools are installed in a bit different way than in Mountain Lion. Evidently you don’t have to install Xcode, you can just do:
I had to work a bit, but GPULib builds again with the new tools.
There are a couple things in general that I am looking forward to using more:
Better handling of multiple displays: you can have a different full screen application on each monitor, you can have a separate set of spaces for each monitor.
The skeuomorphic Contacts and Calendar applications have a cleaner design. Getting rid of the leather texture in Calendar was worth upgrading.
More efficient power management: I haven’t really noticed any differences, but I am really hoping for a bit better battery life. But I’m still having trouble using CUDA with the discrete graphics card on my laptop, which I’m thinking has something to do with the power management features.
In general, I think Mavericks is a lot of incremental improvements, which is exactly what I want.
UPDATE 10/30/13: I have noticed that X11 is always listed as using significant power. Updating to the development build for XQuartz 2.7.5 seems to fix the problem.