Every army has strengths and weaknesses. Successful generalship is often nothing more than arranging the match ups so your best units fight their average units and the opponent's best stuff is neutralized.
This requires knowledge of what is the strength of the opponent army and what they might do with these units. From my extensive play I have learned what pitfalls to avoid and what units to fear.
Since coming out officially in the annual the wood elves have reappeared on the battle field in a big way. With their plethora of shooting and special units they preform like no other 6th Edition army.
While they seem to be a shooty army most wood elf players plan to beat your units in hand to hand. They do this with the Dryads which are combat beasts. With the right aspects they can attack with 3 S4 on the first round for each model which is more than enought ot compensate for the lack of ranks and a banner. Then their fear will eventually win the combat.
By comparison the wardancers are the weak little brothers which seldom win combats by more than 1 or 2 and the treefolk is just worthless. After the Dryads the other best targets in this army are the waywatchers and the archers as nothing else is of impartance. They are very vulnerable to heavy armorered units, like knights, but lighter armorered horsemen might want to hide atleast until the hail of Doom Fire Arrow is used.
The new Chaos book has dramatically increased the variety of Chaos armies being played. Despite this there are a lot of similarities which can be drawn. In all cases the Choosen Knights are as brutal as the old Knights. They can beat units by themselves but their cost and speed makes them very beatable in any flee and flank situation. The chariots are also much better than expected and can often hold up smaller units like a swarm. Chaos Warriors just do not beat anything but are very hard to wound and form an unmovable rock in any battle. Perhaps the biggest problems with Chaos are the characters and their magic. The Chaos Lord is a beaters of the first degree who can often defeat vampire lord is a fight, but Chaos Lord is no match for a chaos demon. There characters can also dominate magic like no other army are Tzench can get 20 casting dice if they want or have a lot of bound spells in the deamons. Either way the secret to beating this magic heavy behemoth is to shoot like a find and hide your weak stuff. The Tzench spells are nothing more than missiles which often will do little damage of a lot of dice. Then when you get a chance try to eliminate units one at a time as fleeing is not good enough. With this in mind try to close fast and wreck some havoc on their weaker units to score a lot of points because of the high unit cost.