For instance, the movement of the UI and crosshair, which both BZ and quite a few FPS players will probably tend to find odd.
For BZ players, the movement will be considered "odd" due to moving independently of the crosshair/target sight.
The field of view in a great deal of FPS games is completely based on the mouse or crosshair.
This isn't too consistent with either BZ or most FPS games, so it the movement of the UI and controls will likely feel unusual to some players.
For future reference though, the responsiveness of the controls may want to be adjusted, as they feel a bit slow in the prototype.
Additionally, the way FPS is integrated could be improved somewhat, although I'm not exactly sure which areas may need to be improved or adjusted at this point in time.