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.

"

) people at this level who still play mostly for tactical tricks, and they work! Granted master level is generally too strong to work against, but below that you can often get away with it if you can see enough tactics. The only thing that can be known about this type of player is that they are more consistent on avoiding blunders or allowing less opportunities to get crushed. Most likely of course they are starting to learn advanced concepts a little better, but if their tactics are very strong, it's an open question on how well they can execute those "advanced concepts" if at all, because frankly they're not needed a whole lot yet.
You are a hero !