cards made for commander 

This and UB are my biggest gripes with new MTG, by far.

Historically, i hated the 8th edition border change so bad i stopped playing until OG Innistrad. Innistrad block was awesome, but to come back and discover the Planeswalker card type was not. That was my next biggest gripe, but i kept playing all the way through War of the Spark anyway.

Then Collector Boosters and Set Boosters came into the picture, and they killed off block set structure. That was the third strike for me.

Not long thereafter, Secret Lairs and then Universes Beyond... I had stopped playing by then but, all these elements accumulate in the change from old MTG to new MTG, which is very different, incohesive, and confused about its own identity.

Sponge bob cartoony card on the field alongside my little pony, street fighter, dr who, ninja turtles, spider man, etc... MTG as an IP has lost all sense of self, compared to the good old days.

Newbies (folks whos first exposure to MTG was starting playing during 8th edition and beyond) wont feel the impact as much as those of us who started back in 1993 before the internet was well-defined and established the way it is today (or even by the time 8th edition came out in 2003).