Crossing

Self-explanatory. Important on 4-4-2 with a tall target man

Dribbling

Self-explanatory again. It's always good to have a man who can get past his personal opponent through dribbling. Normally this should be your playmaker, or a technical  defensive midfielder. Agility and Technique are a must. Be careful. If he is very low on teamwork, this might mean that he might only dribble for himself

Finishing

And again. The most crucial attribute for a striker. Couple it with off the ball movement and balance, so that he is able to receive the ball and place his body correctly, otherwise a 20 in finishing won't serve.

Heading

Crucial for both central defenders and strikers (not owen-type of course). Usually coupled with jumping and strength.

Long Shots

Especially for a playmaker or a defensive midfielder coming from behind, when your frontline is blocked.

Long Throws

Can be good for a fullback, if you set his instructions to take throw-ins. Not an important attribute. 

Marking

Crucial for a central defender. If he doesn't score good in this, don't buy him. If you have to use him, try using a sweeper.

One on Ones

Important for your striker, especially a fast one, who will have to break the offside trap and beat the keeper.  Couple it with agility and obviously finishing.

Passing

Important for central midfielders. Use short passing.

Penalty Taking

Nothing to explain here. Make sure he has good mentals as well.

Set Pieces

Corners and Free kicks. Long shots and crossing also help.

Tackling

Crucial for defenders (central) and DM. Couple it with strength, marking, bravery.

Technique

Technique is different from dribbling in that it expresses an attacking player's natural skill. A technical player will be more apt to control the ball and will give the impression that he feels easy with the ball in his feet. Your attacking midfielders will mostly benefit from this