ETBs are one of the most undercosted things in the game, and copying a good one for two blue mana is a steal
If your opponent discards a card while you control Leyline of the Void, abilities that check whether a card is discarded (such as Hollow One, or a madness ability of the discarded card) still work, even though that card never reaches that player's graveyard
The blow out it when they have instant effects like tap, bounce, destroy etc where they can get a two for one at your expense
Not a great rate as a combat piece, the rest of the text box has some potential
How about a cycle of elementals whose ETBs provided huge immediate battlefield impact, suffered from no timing disadvantages, and cost ZERO mana to evoke
Repeat for infinite damage