Turning in too early. This happens because you think you're going to get through the corner faster because if you turn in early you're not turning in as tight, but by doing this you wind up sliding through the apex to turn out. Your speed through a corner should be "slow in fast out."
Entering the slower corners too fast, then scrubbing off speed as you squeal all the way through the turn. This may sound like #1 above, but it's not. Most new drivers are too fast into the slow corners, but too slow through the faster ones.
Braking while turning in. It is a better learning experience to brake too early and be on a constant throttle well before turn in. As you progress on later laps you can slowly move your braking zone closer to the turn in. This is especially good for the faster corners.
Not looking ahead to your next reference point (apex, turn-out, etc). Know where you are going next to prevent making any mid-turn corrections. Just before the corner turn-in visually pick up the apex. When you get almost to the apex look for the turn-out point. Hand-eye coordination is what's going on here. Look where you want to go.
Not using the whole track. Most of the first-timers quickly forget about hitting the turn in, apex and turn out cones. You shouldn't have more than a couple of feet between your wheels and the berms on the higher speed corners. Many inexperienced drivers will be eight feet away. On the slower corners actually being slightly on the edge of the berm may be good. When you pass by one of these points take a quick glance over to see how close you are. Sometimes having a rider or someone following helps critique this.
Coasting. Either be using the brake or the accelerator. Coasting means indecision because you haven't planned ahead well enough.
Trying to be fast right away. Start off by worrying about technique and smoothness. Speed will come later. If you start off wanting to be the fastest car out there after a couple of times at the track you'll get frustrated by a lack of progress in your times. Inexperienced, fast drivers are usually very unbalanced, choppy and very rushed in the driver's seat.
Frustration. Even though you are doing everything you've been told, you still feel like you're getting slower. If you practice doing it right after a few open-track or autocross events you'll come to a point where you think you've gotten slower, but you're actually faster. That's because if you're really smooth and anticipate your next moves then this lack of hurriedness on your part will seem like you're slow, but you're actually just better!
Communication On the track, the NWARC wants windows down, to facilitate communication between cars and your own car. Passing is indicated by hand signals, not turn signals or flashing lights.