I used to play decks with Megrim in them a lot. I probably wouldn't do that anymore because there are just too many better strategies now. Waste Not is pretty comparable to Megrim and I'm guessing Megrim was kept in mind by whoever was in charge of balancing this thing. I'd hesitate to use Waste Not even more than I'd hesitate to use Megrim, despite it being one mana cheaper. One reason is that Megrim has viable combos, such as with Urza's Guilt. But the main reason is that Megrim is easy to account for. As long as I can predict how many cards I can make my opponent discard, I can estimate how much damage Megrim will do. The semi-randomness of Waste Not, based in part around the composition of my opponent's deck, makes its effects hard to account for.
It's easy to think of scenarios where Waste Not works really well. Turn one, Dark Ritual into Hypnotic Specter. Turn two, Waste Not, attack. Random discard gives me a zombie? Cool. More beats coming right up. Random discard gives me mana? Uh, I'll play Hymn to Tourach to spin the wheel of discard two more times (that's a lie: I actually play Sinkhole and bask in your misery, but maybe I'm playing a themed discard deck for kicks or something). Random discard gives me a draw? Card advantage +2 bonus sounds pretty good. All nice things to have. Not knowing which one will show up next makes it like a special birthday surprise enchantment, or something (although if you use surgical discard spells, like Duress, you have some say). But it's not the good times that concern me. Lots of cards work well when times are good. If I am losing and I topdeck this thing, I'm probably not happy about it.
I wish they'd just stuck with one effect, rather than going overboard and making three separate possibilities depending on the card type of the discarded card. It affects deckbuilding. For example, I might have a monoblack control deck that I think is a bit short on creatures, so I put more creatures in. I play Waste Not and it gives me even more creatures than I'm what I'm already drawing, because I keep making my opponent discard creatures. My opponent plays a game-breaking spell of some sort, and I don't have any answers to it because my deck has such a high density of creatures, and the enchantment I played is only giving me more of them, when more creatures won't save me. If I'd known about that, I'd have put fewer creatures into the deck because Waste Not was going to give me some anyway. But that's not quite right: Waste Not only does that when the cards my opponent is discarding are creatures, and that won't always be the case. Because of the semi-randomness and the increased relevance of my opponent's deck composition, my capacity to plan ahead is reduced. I try to avoid cards that make it harder for me to plan ahead. Planning ahead is hard enough as it is and I don't need some uppity card making it even harder.
I'm making it sound like I think Waste Not totally sucks, which isn't quite what I really think. I think that because it's unreliable and there are so many other options for two-drops, it probably won't be relevant in any constructed formats (I am spewing so much hot air when I say that with regard to Standard, a format I've never played in my life and have only a passing understanding of, but I'm probably right anyway), which could actually be good news if it turns out to be true, as that should mean it'll be a dirt-cheap rare for players who want to craft some weird new discard-based archetype in casual play.