I agree with many of these posts on airport terminal cleanliness, however there are a number of things that should be taken into account when discussing STN or any other airport which experiences significant numbers of LCC's.
The airlines need to utilise the aircraft at least for 3 x rotations a day to make it work, so this means departing early so as to return late evening. This inevitably results in peak numbers of departing passengers going through the terminal between 0400-0700 period and arriving late evening.
Keeping airport terminals clean is a major challenge and much is done to try to achieve this while keeping it operational. Unfortunately many of us humans don't think about keeping public places clean and dump rubbish all over the place without any thought for others.
I think STN does well to do over 21 million pax a year in one terminal and all the staff involved should be praised for what they do.