They are going to divide your current xp by 10. So instead of having 17000/26000 you will have 1700/2600.
Personal milestone. I feel pretty good about myself, knowing that I have hit both the 16 cap and the 21 cap, soon to be 26 cap as well. The banner is nice, but I usually only use the lv16 banner because it's rarer than the newer ones.
And who knows, STS may come up with some bonus for people who capped.