My favorite feature so far is the fact I can switch the editor to release mode, which produces optimized assembly code, but I can still run the profiler on it. That means I get the same timings I would get with a full build, which takes my PC about 6 minutes to make, so I don't. I've been spending a lot of time optimizing and cleanup up lately, so a very nice feature.
I just finished this one, which is a big win:
Optimized terrain control textures to not update with every mesh update.
Those peaked at 2ms spikes on my PC, on several frames, when new chunks load, adding to the FPS drop that already happens when building meshes.