Yikes. This presumably also takes subtypes with it, and therefore prevents not just "Search your library for a basic land card" but also "for a Goblin card" or "for a Forest card". And it's not just being Mindlock Orb - it also hoses a wide variety of other things: Oracle of Mul Daya, Bioplasm, Call of the Wild, Galvanoth, and so on. Abundance and Erratic Explosion stop working; Avenging Druid mills your entire deck.

More problematically, Destroy the Evidence / Telemin Performance / Trepanation Blade / Mind Funeral mill the opponent's entire deck; and similarly Goblin Charbelcher was kinda already broken, but now it stops caring how many lands you have in your deck at all. Recross the Paths now reads "stack your deck", presumably to put your Mind Funeral on top.

Hmm, what happens to Chaos Warp? If this Forest isn't actually a land card any more, then presumably it's not a permanent card either, so this turns Chaos Warp into an instant Vindicate.

Other fun things: Omen Machine will stop giving you lands (you'll try to "cast them if able", but you won't be able).

Ooh that causes a lot more problems than I realized. I need to either change the effect, limit it to graveyards, or make it cost more.

I guess it depends on whether you want it as a hoser or a "hmm, this is a hoser, but oooh, I dcould do bonkers things!" piece. I'd much prefer the latter, but presumably you saw a need for the hoser?

I did mean it as a hoser, but I hoped it would have some other cool interactions. Which it does, obviously.