Stat advantages through levels have been smoothed out.Honestly speaking, Leona + Graves will still beat the stuffing out of you at level 2, but level advantages should be more focused on ability ranks than raw stat gains.
Given how strong basic attacks and ability levels are during the early game, flat stat per-level advantages only magnify early game snowballing - especially that level 2 power spike that laners use to punch their opponents in the face.
For the attack damage change, we saw an opportunity to lightly address the minor imbalance between ranged and melee in the solo lane, so ranged champions (and Jayce / Kayle) won't have their base AD modified by the base stat buff (which rounds out to +2 AD for melee champions).
There's something to be said about loss aversion, but let's not go into that.
These mana regen changes are less related to stat gains and more to do with our modifications to mana regeneration items (specifically how they're now based off base stats rather than being flat values).
There's a lot of math behind the curtain, but this also sets the foundation for any future work we do on differentiating champions through on mana regeneration.
In other words: before this patch, mana regeneration was a little arbitrary, with each champion roughly balanced off their closest counterparts.
Now if we do change champion base mana regen, it'll be for a very deliberate reason and you'll see clear groups of champs affected differently by mana regeneration items.
While we're aiming to improve the quality of life for many junglers in the preseason, the truth is some junglers are going to get more mileage out of these changes than others.
While it might be weird to get "pre-emptive" nerfs based on systems that haven't been publically tested, these guys and gals (and mummies and rammii) just keep coming up in our internal playtests, and we don't want to be responsible for Amumu (just an example) becoming more of a solo-queue star than he currently is.