A graph of the last 24hrs and the last 7 days of the state of any device would be really helpful in debugging.
You can see if a light is on when it should not be really easily on a graph.
Also really handy to check that your thermostat setbacks are working as expected by viewing a graph of the current temp. vs. setback temp.
If you put the data into a MySWL DB it should be easy to generate some simple graphs.
Ideally have a choice of line vs. bar chart is also helpful but start with simple line graph.
Having more choices for time duration would be nice, but last 24hrs and last 7 days is fine for a first step.