I don't mind background music as such, although it can sometimes be a bit heavy-handed. I dislike it where it is so loud it obscures the dialogue (this also often happens with radio programmes, which is inexcusable).

I dislike it where it is incongruous or simply poorly thought-out. Last night I happened to be watching Timothy Spall sailing around Cornwall. The producer had apparently decided that something folky would be appropriate, but rather than something with nautical or local references the tunes they chose were either morris (what does 'Shepherd's Hey' have to do with boats?) or Irish (what do uillean pipes have to do with Cornwall?). The disconnection between the music and the programme jarred.

And why do they never credit the musicians?