"Riot's gutting mana regen"

No they're not.

Is it a nerf? Sure. Is it a big nerf? Let's see:

Instead of 3 mana per 5 seconds, you'll now be able to get 75 mana (100 if you spend an extra point). 3 mana per 5 seconds provides this much mana after 125 seconds of being below full mana, so about 2 minutes. For time after this, it provided 36 mana per minute. So, how long are you out in lane? 5 minutes in lane, and you're down 105 mana compared to the current mastery tree. How many spell casts is that? Less than 2? Spending longer than that in lane? What items are you giving up by letting that gold sit in your pocket? (Also, as a smaller point, blue buff will be stronger with the new masteries, almost making up the difference when you have it).

Again, I agree it's a nerf, but let's not pretend the sky is falling.