The real reason behind Karthus nerfs
Isn't because of the new masteries or anything. The real reason is because he synergises very well with AP 
. And the koreans were spam picking them with a 77% winrate when both were in the same team.
AP deals massive %health magic damage with his passive, it's percentage based so building health vs him is futile. To counter the threat of AP korean teams started building magic resistance vs him, effectively reducing the impact of AP in the game.
But the AP abusers had to find another way to get freelo so they discovered that 
wall shreds magic resistance, effectively countering the only counter to AP , letting him instakill tanks with one basic attack again.
**
"But deals magic damage so MR would be a counter to both him and AP !!!" **
This is were AD came in. The Koreans discovered that AD were much stronger than AP , mainly because of his synergy with trinity force, statikk shiv and rapid firecannon and started building him this way, making it impossible for the enemy team to counter AD & AP .
Riot then saw the popularity of rise significantly amongst Korean teams and thought he was OP and nerfed him, ignoring the real problem that is AP .
For more information see:
http://boards.na.leagueoflegends.com/en/f/3ErqAdtq/d/2bBE4w8M?comment=0001
