A 10-point achievement entitled "A Primer In The Magical Arts" pushed me over the edge.
Even though I'd received a number of recommendations on various one-point achievements that would have nudged my gamerscore from 19,999 to 20,000, I didn't follow through on them. Like I said yesterday, achievements mean little to me (i.e. screw you, Assassin's Creed II feathers).
Still, it's a notable milestone. Onto...30,000?