AD items have more build diversity than AP items ....
AD items can be subdivided into various categories. For example, you have marksmen items, assassin items, and bruiser items. However, when you take a look at AP items, the only category is pretty much mage items. There really isn't much diversity. A lot of items simply offer ap/health and a lot of their passives/actives are a bit dull with the exemption of a few such as...
(Rylies is getting nerfed)
If you compare that to ad we have

From an AD stand point, the only AP item I would envy is 
From an AP stand point, I envy

AD items offer movespeed buffs, autoshields, range enhancers, CC shields, Invisibility damage proc, damage reduction, lifesteal. Whereas AP offers stasis, mini dash, and lots of health......
I feel like there is so much more creativity in AD item actives and passives for both defense and offense. Whereas AP, defense means health and zhonyas. AP offense only offers AP stats and the only interesting AP offense active is 
I feel like Riot needs to have another look at AP items because, out of the marksman/mage/assassins rework, AP items drew the short end of the stick.