It's Donna-lite because they needed Catherine to start shooting "Turn Left" which is amusingly the Doctor-lite episode. "Midnight" (next week) was their cheap episode of the season: one set, limited f/x etc. I believe it only took 8 days to shoot whereas "Turn Left" (the following episode) is very very complicated and will be extremely Donna-centric.

If it makes you feel any better, DT said he thought "Midnight" was his fave episode of the series because it was shot more like a one set play than anything else. Apparently the takes are very long and it's more people interaction than anything else.