This is what is known:
There will be mini expansions and Major Expansions, similar to the way Burning Crusade came for WoW.
As for DirectX 10, i think people here are misinformed. According to Games for Windows and Wikipedia, GW2 WILL support DirectX 10, HOWEVER, that does not mean it will not support DirectX 9. It is as the developers said in PC Gamer May Issue:
" Our Goal when writing the Guild wars engine was that it would run well and look great on the majority of Gaming PC's, even those that are 2-3 years old or that don't have the latest video cards, We think that one of the reasons that Guild Wars has enjoyed worldwide success is because almost anyone who plays PC games can play it. We have identical goals, if not higher, for the GW2 engine. We want it to work well on most gaming systems in the world when it's released, and to scale well so that players with high-end hardware get to see more stunning details and visual effects "
So what does that mean? It means it WILL run on your P4 from a year ago, but if you have a 8800 Ultra with Quad-Core processor and 4Gb of RAM, you will fall in love with GW2. don't forget that GW2 comes in 2 years, which is MORE than ENOUGH time for DirectX 10 to prove itself and video card prices to fall.
http://en.wikipedia.org/wiki/List_of...X_10_support#G (First link supporting my DirectX 10 comment, searching for more...)