It is the first time that booster packs contained 15 cards, as we know them today
The flexibility to not only copy your own spells but opposing spells as well means you should always find a good target, whether it be removal, card draw, or anything else
As a baseline example of how messy this can get, imagine playing Dark Depths and sacrificing it to, say, Crop Rotation
Agadeem's Awakening is also obviously very good in commander and Mono Black standard lists
the majority of the time, the answer is to make room. When its particularly relevant, Ill mention it
You can make infinite tokens with Appa, Steadfast Guardian and Felidar Guardian, for instance, or draw your deck with Whispers of the Muse, Chakram Retriever, and a mana dork