The current mastery system is a limiter on in game choice, not a multiplier.

Simply put, because so many of the masteries are focused around giving you more passives that scale off very specific criteria (attack speed, attack damage, crit chance, Bonus Hp) this means that optimum builds are even more restricted, based on what you selected before the game started. You are now doubly punished for being forced into a sub optimal build path so the correct response in many situations when behind is to simply keep building towards your optimal 6 items and let the enemy keep killing you and hope your team can somehow collect on the shutdown bounty.

If you have built into either of the offensive trees, there is no point in simply "going tank" because it might be the right thing to do. You will be a bad tank and you will be wasting your money because your masteries do not support a tank build. Hilariously, the best "tank" keystone (grasp of the undying) is more suited to an adc or ranged bruiser like Nidalee or Gnar who can proc that intensely obnoxious passive every 4 seconds without any threat to themselves whatsoever, but that's a rant for another thread.

Anyways, the TLDR I guess is that we actually had more meaningful choices as to item build paths when runes and masteries were just lame stat boosts.