Isochron and Lightning Bolt allow for an instant win turn 5 as long as you have non-land mana generation like artifacts, cryptolith rite, or mana dorks. It's only slightly more tricky than the splinter twin decks, but even that had some weaknesses this one wouldn't.
I think you'd be able to fit the infinite damage in the same deck without making it convoluted. Isochron and a lightning bolt or Giant Growth can function on their own without going infinite.
They probably had the whole block templated already and sent it to print before cramming the rest into one set instead of 2. Still would have made more sense to do colorless a set earlier, but I'm guessing this whole block is messed up due to the last minute change in block structure.
No love for Woodland Wanderer? Essentially, when paying the Siege Rhino cost you end up with a 5/5 Vigilance Trampler. In a format with tons of scions chump blocking, and few trample abilities, that has to mean something. Plus, if you're splashing other colors or get mana screwed he has the upside of scaling to your manabase.
Also, I doubt big drop eldrazi will be as big of a thing as lower cost ones with 6 mana being the top of the curve. It would be easier to overwhelm with numbers than to drop a giant and have to break through scions each turn first.
EDIT: Or remove isochron and use Sprout Swarm, Haze of Rage or Wurmcalling in combination with cryptolith rite.
Or guttersnipe and any mana-producing spell attached to isochron.
Also, I doubt big drop eldrazi will be as big of a thing as lower cost ones with 6 mana being the top of the curve. It would be easier to overwhelm with numbers than to drop a giant and have to break through scions each turn first.