The fact that it does not have Flash was pointed out by many as a major downside, but I think that it is simply a necessary way to balance the card, as it would be too strong otherwise
(the last top 10 had Insidious FungusInsidious Fungus in this spot)
We have a lot of the main tribes in these colours that work including goblins, saprolings, zombies, vampires, and I am sure many more
[casthaven]The Abyss[/casthaven] will slowly pick apart any creatures that your opponent is running with the exception of artifact creatures and those with Hexproof/Shroud/Protection like [casthaven]True-Name Nemesis[/casthaven]
Protection is a powerful ability for a creature to have, much less a creature that costs two mana
In-game references[] As one of the most famous cards of all time, the Black Lotus has inspired numerous cards with names, art, and abilities that reference it in some way