New year, now I'm playing with the most basic stuff: global effects. This time I'll try to be more patient and take a more systematic approach, instead of the "hack anything that looks promising" approach I've been taking.
Right now I've a nice number for my qualifying RMSE: 0.8888. Hey, it takes real skill to hit such a nice number by chance ! I'll try to resist the urge to submit better results until I'm sure I've made a big improvement.