Everywhere in the worldspace, there's loads and loads of different coloured boxes. These were not in the previous CSs. What are they?

Lighting, smoke and whatnot I assumed. Fallout had loads of them.

They could be roombounds, triggers, or audio volumes, depending on where you are and what settings are on. You can hide most markers by toggling the "M" key, using the Show/Hide menu, or by selecting the individual object and hiding it with two taps of the "1" key.

If you hide objects with two taps of the "1" key, you can get everything visible again by pressing the F5 key with the Render window active.

Alt+1 is a new alternative to F5, as well. F5 forces the cell to fully reload, whereas alt+1 only forces hidden objects to display again.

Good information to know, thanks!

I have noticed on occation that F5 will make most of the cell actually become invisible. I fix this by going to a different cell, and them going back to the cell I was working on. If that fails, I reload the CK.