They made a powerful combo deck that was difficult to interact with, causing Wizards of the Coast to perform a suite of bans that took almost all of the combo decks out of the format
However, it can't attack or block unless it has an even number of counters on it
To make the most out of March of Souls, don't look at it so much as a sweeper, but rather a form of mass creature-replacement
This answer makes it seem as though it could be a valid choice to randomly decide to have more than 60 cards in a Constructed deck "just in case"
The deck is incredibly strong and, thanks to Murderous Rider, Dark Salvation and Languish, has fairly good removal as well
There was an outpouring of positivity, one reply saying "If someone did that for me my heart would melt into a puddle on the floor", and many users apparently turning green with envy