Originally posted by brown29knight:

all of them. iirc, every skill loses 5% of it's value, round up, so every skill will lose 1-5 points.

(community, feel free to correct me/clarify if I'm off on some details)

Yes, every skill loses 1 level per 20 gained (rounded out). You can check your current skill levels by pressing tab (kb/m) or Y (controller) to bring up the inventory UI and clicking on the three-triangle symbol on the upper-right corner of the screen.