Dredge decks use the dredge mechanic to quickly fill up the graveyard
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Mental Misstep was banned in the initial Modern banned announcement, based mostly on how it played in Legacy, where it essentially pushed one-mana plays out of the format and made most decks 56 cards, since everyone had to run Mental Misstep, not so much because they wanted Mental Misstep in their deck but because they needed to be able to counter their opponent's Mental Missteps
Aragorn can share counters with other counter types, too, such as widely-used +1/+1 counters, but green and white have other, better options to give +1/+1 counters to all their creatures and pump the team
Casting a Mulldrifter for 7 mana via the offspring mechanic means that you get an extra 1/1 and two cards, as a tame example