The only time I've ever had a bag delayed was when I barely missed a connection on US PHL-LGW and was rerouted on BA to LHR that left within twenty minutes of the US flight (so I didn't expect the bag to make it).

In contrast to what you see on the news, lost bags aren't a big problem (unlike poor reporting). Pack what you need for one day (medicine, toiletries, a change of clothes that is appropriate for the weather or what you'll be doing where you're going) and don't worry about it. It's actually pretty nice to move through the airport unencumbered.