Mystery might be fun, but I'll lift it: you get a minor amount of XP just for logging in consistently, and you get an appreciable amount of XP for voting. Someone once calculated that if you pop in and spend all your votes every day, you can become a saint in 8 months. Without ever writing a single node.

That is no flaw in the system, btw: voting contributes to the site, and that's all XP means.

It's also no flaw in the system because it's not particularly effective; you will gain more XP than you can get for a day's worth of votes from a single decent node. People have gotten to saint in about two months without any special effort. If that really was your goal, you could probably get there in two weeks just by writing a bunch of somewhat well formulated, subsequently frontpaged Meditations.