Adjustment layers

FCPX doesn't have them. You can't slap a layer on top of a bunch of clips to apply filters or colour correct across all of them.

 …which is a pity, because it would be terribly useful.

 …and is also not true, because you can. You just slap an empty title caption on top, then apply colour correction and bung filters on to your heart's content. Here's how:

How to build and use adjustment layers in Final Cut Pro X - fcp.co

I've just done this to slap a white balance correction and vignette across an entire scene, then tweaked colour on a shot-by-shot basis beneath. The biggest issue is that in the Project browser you only see the adjustment layer preview default, not thumbnails of the final composition.