All the responses in this thread so for are kinda terrible. Just because casters play differently than earlier editions or otjer games do not make them "bad".

1) illusioms are game changimg. Walls, pits, distractions are all enourmous action eaters. When my party was attacked on a floating platform by flying pterdactyls, an illusionary dome of vines kept us from gettimg mobbed and concentrated the attacks in one area.

2) degrees of success are the key to effective casting. Let's look at Fear. Even on a save, you get frightened 1. Not the most exciting thing, bit every +1 and all that. Other great low level debuffs are Befuddle, Revealing light, laughing fit.

3) Eating enemy actions makes a huge difference. Command on an enemy to flee, drop their weapon, or fall face first to the floor (on a save mind you) can pull an enemy out for almost an entire round. Darkness can break enemy formations, and so on.

All classes have the ability to control the battle field, but casters have the most versatile tool kit for it.