I'll be honest, the people were a lot more fun to play with. People tried new things, were friendlier in general, and every game was a unique experience. Now it's just annoying assholes spamming the same champions with the same cookie cutter builds every single game. People played to have fun and win, not just to win.
No game is safe from the 12 year olds.
Anyways, the item I miss the most is probably DFG. Luden's is great and all, but nothing replaces the burst and cdr of a good ol' DFG. There's also a bunch of items I really don't like in this game now. Blade of the Ruined King, new Black Cleaver, and Titanic Hydra to name a few.
Champion rework definitely Maokai. I can't stand what they did to him. A lot of you probably don't even know he had a minor rework done, but he did. Back in the day I always had a blast playing Maokai as an AP mid. He was immovable, insane wave clear, high burst, still quite tanky despite building glass cannon, it was the perfect counter to the popular mid laners at the time, namely Kat and Akali. Now he's sacrificed damage for a bit more CC, lost range on both his W and ult, and just isn't nearly as fun.
I miss old Twisted Treeline. The new one is pretty much complete garbage. A failed experiment. The old one definitely wasn't perfect, but it was a fun break from 5v5s.