Silver is absolutely right about the grip.
Turbine, I feel your pain, especially the more toward a vertical cast you get to and the farther you are trying to cast - ESPECIALLY if you have a long casting stroke and drift backwards a long way after the backcast..
It is very easy to illustrate with an overhead cast for someone with a long casting stroke. Forget the rod, just stand with your legs apart and arm fully extended with your hand a fist and your thumb straight up.
Now, keeping your thumb straight up, draw your arm back past your ear and, still keeping your thumb straight up, extend your arm behind you till your elbow is straight WITH YOUR THUMB STILL FACING UP.
It is physiologically impossible so long as that arm has a radius and an ulna. The two cannot twist around each other. It is not possible to use a thumb on top with that casting style for the backcast.
So do it again with a palm forward grip. Just leave your hand open palm down extended in front of you and repeat the proceedure so that you stop with your elbow straight and your palm facing up.
That is why Distance casters like Chase Jablonski who prefer a thumb on top for the money cast, but use a 170 degree style casting stroke, change their grip duing the "pause" from thumb on top to palm forward and vice versa.
For shorter casts where your hand doesn't go past your ear much - the thumb on top is fine (though newbies tend to drive the line into the ground as Silver said).
I find just the opposite of what Lefty said about the thumb on top curing the 'wow' on the back cast.
Exactly. It is not possible with an overhead cast. It gets much easier as you move more and more towards total sidearm (rod parralel to the ground).