Trying out a cave wall, I sometimes accidentally push verts off into the distance. Having a gizmo with 3 axis and 3 planes would be really handy at times. (like this:
http://www.finalclap.com/ressources/images/3dsmax-tracking6.jpg )
The gizmo is one of those things I often lack in level editors for older games (most newer editors have them).
alternatively you could try having cubes for handles, and selecting the y-face on the cube would move on the y-plane, etc... you could have a similar system of bounding boxes when selecting an entire brush.
(on a sidenote, in the view tab, having the ability to hide vertex handles on backfaces could be useful)