User blog comment:TheNewBuildGuy/Possibilities-- Jungle Zyra/@comment-99.187.230.227-20130228065816/@comment-70.61.75.234-20130228175040

He'll only do 800 bonus damage on hit after ten seconds. His range is 475, which means he's in easy poke range during lane. His attack animation is horrendously slow, so you're locked in place for quite some time while last hitting a minion. He has no poke of his own for lane, or late game when sieging down a tower.

Now, AD thresh is a thing, but not ADC thresh in bot lane building attack speed and damage. Top lane he does well against weaker opponents lacking strong ranged harass or multiple gap closers. An early heavy AD item like BT gives him the ridiculous scaling on his Q he wants, and he can build into health afterwards to be a beefy intiator late game.