Out of experience, I'd say class A players are fairly good as long as they're not dragged out of their comfort zone, while expert players have a larger array of chess knowledge and are able to play decently a larger number of different positions. This is even more true when you reach master level, and you see for example people able to play a very large # of openings and positions-types.
That's for the breadth part. As for the depth, class A players don't blunder too much, know all basic tactics, can calculate correctly and play according to the position, albeit in a limited # of positions-types they're familiar with. From a sporting point of view, they may crumble easier when faced with difficult opposition. Their endgame knowledge may be very limited (I speak from experience).
I train with a Class A player. All the above looks 100% correct, except for comfort zone. I've seen him play with so many different openings. It appears like he knows all of them to a certain degree and, I know from experience that he knows some of the openings from beginning to end.
"Know all basic tactics."
I made tactics priority number one for 2011. That means, if I only have time for one thing to do in chess on any given day, it will be tactics.
Here's a list of tactical motiffs found here: http://chesstempo.com/tactical-motifs.html
Each motiff has a diagram underneath, teaching each tactic with text and piece movement.
The tactical motiffs: Advanced Pawn, Attraction, Back Rank Mate, Blocking, Capturing Defender, Clearance, Discovered Attack, Distraction, Exposed King, Fork/Double Attack, Hanging Piece, Interference, Overloading, Pin, Sacrifice, Simplification, Skewer, Smother, Trapped Piece, Weak Back Rank, X-Ray Attack, Zugzwang, Zwischenzug.
You are a hero !