The biggest change is the forthcoming ability revamp. According to the notes, "almost all abilities have been changed, with the goal of increasing the number and viability of different player roles and play-styles. Some abilities have been removed, and some new abilities have been added." Patch 1.8 also introduces new NPCs and missions in various parts of the wasteland, UI tweaks (including new chat channels), and an adjustment to the rate at which random AP is gained.
If you've never tried the PTS on for size, you can download the client and also pay a visit to the forums to get your bearings.
