I was really hoping to see Mordekaiser in Worlds; Patch 5.18 killed that.

He's now nearly unplayed in the pro-scene - its not that he's under powered per say (he's around a 50% win rate as ADC)), its that the pro scene highly values reliability, and Morde now has no fall back pattern - He can't shove waves rapidly and heal with his w anymore (net heal of 10 hp level 1, 20 hp at level 5, from 50 to 130).

Furthermore, his once strong early game is now terrible - his W used to have high base damage, but it was very close to halved (380 to 210 level 1, similarly nerfed at higher levels as well)

Now competent players poke and zone him, or just flat out all in him - He's among the squishiest champions in terms of HP and Armor, and he has no way to get away. His strategic niche of brutal all ins early game is now far better preformed by other ADC's such as Draven or Graves, or nearly any other melee bruiser.

The bonus XP for last hitting matters very little in high tier play - He's likely going to have trouble last hitting in the first place against many match ups, but more importantly, ADC's get quite a bit of solo lane XP in the first place as supports roam and ward - and while Morde's support is roaming, any ADC or support with good vision will zone him out of XP range altogether. He has no tools to deal with competent ranged champions on his own.

Sure he has some gimmicks (Dragon's pretty cool), but he's neither reliable or particularly useful any more.