There are three types of cards you want to mill for.
A: Cards that Activate in the Graveyard, or otherwise give some kind of benefit when they are in the graveyard.
B: Cards that need to be in the graveyard to activate other cards effects.
C: Cards that need to be in the graveyard to fulfill the activation requirements of other cards.
Examples of A include Necro Gardna, or Vayu. Subtypes include Mezuki and Sirocco the Dawn, because they arent exactly useless when you have them in your hand.
Type A you usually want to focus on sending to the graveyard from your deck or otherwise. Most of the better ones being Dark really help thanks to Armageddon Knight and Dark Grepher
Examples of B include Pot of Avarice, Monster Reborn, Illusion of Light, or Call of the Haunted. These cards tend to be better the more filled your graveyard are with monsters.
Examples of C are Judgement Dragoon, Rainbow Dragon, The Dark Creator, Dark Armed Dragon, cards that count the number of monsters in your graveyard in order to be activated or summoned. Another card that falls in this category is Magical Explosion, with counting Spell cards, as opposed to monster cards.
Now, with the release of Charge of the Light Brigade a week ago, it seems that the "Oh lets just jam in Charge, Rykos, and some Recharge to mill" has become the main way of sending cards to the graveyard. Not to mention it mills without losing advantage.
However, in decks that have cards they want to mill and dont want to mill, this becomes a problem.
For example, the decks which are said to be the best for milling, Vaayu Turbo and Undead Synchro, have strong cards that are probally a very bad idea to mill off. For example, Burial from a Different Dimension. If you activate charge, and mill off a Sirroco and Vaayu, but the third card is burial from a different dimension, you will end up losing advantage.
Especially for decks which have strong key components for the win, such as AKB or Cat Synchro, milling off a Cold Wave, Pot of Avarice, or Heavy Storm will really make or break the match.
So, before you randomly splash some Charge of the Light Brigade if you have some cards to mill, think about:
1: How many cards do you NOT want to send to the graveyard?
2: If that card were to go to the graveyard, how would you follow up and get it back?