The story begins when you first find lower deck overcrowded and hear driver continuously asking passengers to move upstairs. Well, that's the chance for you as an impression greedy visitor and you take the stairs looking forward to experience a bit of London romance.
And these expectations to face something new turn out to be truth, because upper deck is really something different. First, usually it is more dirty there. While food scraps, unfinished drinks in deformed cans and paper cups, etc. are rarely seen below, on the upper deck it is usual for the rubbish to be found. Second, things are less private there, I mean, people don't care that much for they being loud on a phone or throwing something to their lads in the opposite corner, and so on.
Third, it takes time to get used to the street view from above - even familiar neighbourhood looks somehow different, with other, previously skipped details cathing your sight, so the stop you need might be easily missed.