Somebody on IRC mentioned today that a rain effect would look nice, so I made a quick one:
Note how the rain is realistic in that it falls until it hits something, then shows a splash. So there is no rain inside the castle or underneath the scenery. This takes a little CPU, so very heavy rain (thousands of drops at once) might slow you down. In the video there are about 1,000 and speed seems ok.
The effect is about 40 lines of code, in library/1_3/CustomEffect.js. You can see it in action if you run the latest git, and play the storming_test map.