We don't know yet (we will probably find out on March 16th when preorders open).

The general theory though seems to be we'll likely need to buy the expansion for each platform (PC/PS3/PS4) and if we don't have an expansion on one format, we'll likely still be able to play, but won't be able to enter any of the new areas (and won't be able to login at all if logged out in a new area, but will likely be okay if logged out in a non-expansion area).

Gear/items acquired shouldn't be affected, just won't be able to get more from the platform without the expansion!

Like I said though, we just don't know yet and this is all speculation, we'll know more in a week.