Firstly, the devs are axing Darkfall's initial client cost. The monthly subscription fee will remain, but it will be reduced from the standard $14.99 (though Aventurine hasn't revealed the final price just yet).
In terms of gameplay, the devs are permanently increasing avatar skill gains. Specific details are scarce, but it's all part of the promotion that will begin on June 27th. Anyone who has ever purchased Darkfall may come back and play for free for two days. On June 29th, the aforementioned client and sub fee changes will debut, and they'll last through the release of 2.0.
[Thanks to Dengar for the tip!]