Since fracas posted his comments I thought I would post mine as well.
1. Magic is a potent addition to an army, the nazgul are particularly good for magic but must be protected. This means that they must keep enemy hero formations at bay, typically with transfix, if that fails one can still move them to a different formation for protection. Black breath is useful on enemy heroes as well, reducing their fight/courage so they are weak and the company can no longer use those stats in combat. Good for keeping most humans from charging terror causing companies.
2. Warg riders are frickin shweet. For 25(20+5) points they can have shields(d5) and they come with throwing weapons(strength 4 btw). Add a captain and they can do an at the double with no penalties(movement or courage) since they would probably be outside 6 inches needed for the 1/2 movement and -1 courage rules. All this and they can still cause damage before they charge the enemy (providing they are within 6 inches for the TW at the end of their movement).
3. I am not sold on bows yet. I think one needs a large formation to make them effective. And once one is able to have a large formation of bows ( high point(s) games )your enemy will have more points anyway.
4. However I am sold on the smaller siege equipment, they can do some serious damage and have the potential to be monster killers.
5. If you have a hero magic user you can perform an at the double to get within spell range. They are able to cast at ANY point in the move phase. Remember the formation uses the heroes courage value for this roll.
6. More than one captain in a large formation is a must since over the course of the game might will be needed to resist magic.
7. I disagree with fracas, Epic heroes can not join the mumak (or any other monster).
8. Ambushers seem like a great way to get in behind enemy lines but most of these formations for good are rare, evil has more common companies with the ambush rule.
As far as mordor goes, Morannon orcs seem to be the best common formation of infantry while warg riders best for cav (possibly morgul knight but for some stupid reason they are allies). Misty mountains are probably going to be a sleeper giant, some weak companies and some regular ones that are still cheap.....cave drake the hero eater...awesome...just awesome. Druzhag, summon beast(spiders are best?) behind/beside your foes. Cave troll, cheap, fast, 3 attacks and still hard to kill.