Dash gives it built-in haste, which means it can slam the door on a game out of nowhere (you still pay extra commander tax when you dash from the command zone)
Fungus creatures synergize well with strategies that focus on token generation, sacrifice effects, and gradual value accumulation, making them ideal for decks that thrive in long games and attrition warfare
Ultimately, any mechanic that alters the fundamental rules of the game across all formats should be considered incredibly carefully, especially when it will drastically warp all future gameplay around a handful of new cards
By value, Inferno X (M2) and Storm Emeralda (M6)
Only time will tell if Series 2 will have the same impact
Rare cards will, of course, be more expensive, but thats hardly the only consideration