The concept of the CET machine is actually rather fascinating. Trist's device doesn't just capture pictures or samples of the planets he's visited - it takes sections of the actual planets and traps them in a machine that, because of faulty design, one can enter and exit the projections of these planets freely. But where does everything go when the machine is switched off? Or when a different planet is selected on the viewing screen? What has Stotz been doing on that small section of planet for all this time?
Also, the fact that the dimensions of the projections have exit points elsewhere on the Empress is a notion that might have needed more explanation. However, imaginative ideas that aren't explained as much as I'd like are much more fascinating that dull concepts that have far too much time devoted to them. The CET projections are just another little unique quirk that has made this story so enjoyable.
Another unique quirk? The Mandrels. Yes, they do look like Muppets with large clam shells for faces, but they didn't half impress me as a 10-year-old. The disintegration effect of one of the poor creatures turning into Vraxoin in this episode is quite impressive, which counters a less than effective sequence of Mandrels rampaging through a seating section on the Empress. The comedy faces on some of the passengers who are apparently being mauled just make you want to laugh at the entire scene along with the (drugged out) Rigg.