Wizards says it brought in Hexproof as a replacement for Shroud, which was deemed too confusing and too commonly misunderstood
like Coercive Portal (and convenient marriages, one would add?), this Ring can also make politics a lot more engaging
In those days when the only delve threat was Tombstalker, before Deathrite Shaman, Ragavan, Nimble Pilferer, and the like highlighted peak efficiency among 1-mana threats, Mongoose was the card that let you slowly whittle down your opponents when your Delver of Secrets and Tarmogoyf were too vulnerable to removal
Blue/white moxen and Mishra's being two main priorities (remember, we dont play Cities so Fellwars wont do shit)
Exiling permanents on cast and threatening to mill opponents out makes it a premier finisher
It stands tall with another member of its cycle from The Lord of the Rings: Tales of Middle-earth , Lrien Revealed as deceptively powerful commons