Literally everything that makes champs deal more damage.
Their base AD, attack speed, stats and damage granted by items, AD and AP ratios. Literally ALL damage done by champs, and keep the damage monsters, minions and turrets deals.
Try this out for at least one patch, and if it doesn't solve any problems, just revert it.
The damage IS way too high atm. It's a game of "who can oneshot the other faster", rather than "use your brain" like it was back in Season 1 when I started playing. I don't feel "outplayed" when an enemy with 1 more kill oneshots me. It just feels frustrating.