I only watch BOTS 2/3 of the time, so may have missed something, but has anybody on BOTS actually said "I don't like the theme", "I prefer previous big brother series", "I preferred the old housemate selection", or anything along those lines?

Even when previous series are mentioned, it's only ever the C5 series. Are they not allowed to talk about the C4 series, very rarely about other C5 series, or to say that this series wasn't the best?

No they are there to discuss the BB highlights on the day they are there .

Everybody who is invited on the show is contractually obliged to say 'This is the best Big Brother ever', or 'I'm loving it'.