Oh, no! Say its not so. Some heretic is desecrating our sacred game by playing just to have fun. What hope is there for this world?
What makes a Class A - Expert Player?

I have found on my own trek through A class, and expert to NM that the biggest difference is that the higher rateds are far more consistent at beating lower rated opponents. Now, I dont mean 10 to 50 points lower rated but lets say a whole class (200 points) or more lower rated. This means they blunder less and less as their rating climbs. I have known experts struggling to make master who would score about 50% with NMs BUT they would still drop games to A players and sometimes even lower , this kept them from the NM title.

He needs to be 6'2" tall, 53 years old, Irish good looks, great bassist, resonant voice, strange sense of humor, diamond member at chess.com, USCF ID#10130158......
Who could that be.....

I have found on my own trek through A class, and expert to NM that the biggest difference is that the higher rateds are far more consistent at beating lower rated opponents. Now, I dont mean 10 to 50 points lower rated but lets say a whole class (200 points) or more lower rated. This means they blunder less and less as their rating climbs. I have known experts struggling to make master who would score about 50% with NMs BUT they would still drop games to A players and sometimes even lower , this kept them from the NM title.
Yeah I think this is the biggest thing. Consistency.
I have found the stronger players tend to draw games they are losing (and win games that are equal) and that this is the added push that makes them a higher class. Also as Reb said-you can not lose games to people a class below which is also the hallmark of the expert/master

From what I observed the difference between a Master playing a Class A player and an expert playing a Class A player, is that the master is more willing to mix it up. An expert will generally stick with his first string openings and try to play very correct chess. Masters are more likely to play second best openings or even anti-positional openings, in order to create imbalances and get the Class A player out of his comfort zone.

I am impressed by the number of non A/Expert players who took part in this thread.
What?? Most of the players commenting are probably between A, and expert class...w/ a Master chess player thrown in the mix. Of course, you could have been kidding

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).
Yeah, a lot of people think that what differentiates a Class A or and Expert player from the rest of the lower rated people is the ability to consistently beat those below them. I would also say that this person needs to be able to recognize tactics immediately, right?
How good a player is depends on how good he is on the different elements of the game:
Openings - middlegame - endings
Strategy and tacticks are crucial in the whole game.
After that you could talk about style, the player who can handle more type of position's know's more and are a better player.

It could be about a stronger will to win or something similarly exotic, or it could just be that an Expert has studied more. For example, if you look at this site:
http://www.kenilworthchessclub.org/articles/index.html
The guy who created this site is an Expert. I find his chess knowledge pretty impressive.
Less mistakes, more opening prep, better at calculation, and probably a bit more seious about the game.
I know many good 2000 players that could been much higher if the took it more serious, but they just want to have fun, in my club we have an 2200 player that totaly gives a dam, playes shitty openings and takes to high risk's all the time, but he is still a 2200 player!