The cool thing about the ability is that the land in question doesn't have to be a basic Island. So if you have dual lands such as Breeding Pool or Hallowed Fountain (or Tundra or Underground Sea) that have the land type Island, you get to freeze creatures for a turn each time you play one of them. Best of all, if you happen to make all lands Islands (such as with Stormtide Leviathan or the Enchantment Prismatic Omen if you're also in Green) every land you play has that effect.
While it's competing for the same space as Icefall Regent in Standard, which can keep a creature tapped down as long as it's on the battlefield, the Guardian of Tazeem can tap down a lot more things. It will be interesting to see if this guy sees some competitive play, because against aggro strategies, this is an extremely good control option.