Much has been made since the pre-season patch dropped about the disappearance of early armor/MR, the sudden increase of early damage, and tanks doing an overabundance of damage because of their high base damages. The thing is, these are all connected. And most people, even those who play the game a lot, don't realize it.
Now, the way damage in this game works, is it is divided into 3 categories; physical, magic and true damage. Armor reduces physical damage, MR (magic resist) reduces magic damage, and true damage cannot be reduced, it affects health directly. The more armor you have, the larger a % of physical damage is reduced, same for MR, but you start to see diminished returns after 100 armor/MR. In the same way, you start to see a larger investment in your damage the lower your targets armor/MR is, with enemies effectively receiving true damage when they have 0 armor/MR.
The graph below shows the relationship between resists and the amount of damage received:
Now lets take a look at how this affects 2 of the roles that people see as the biggest offenders of dealing too much damage with minimal investment into it; tanks and lethality users.
Now, tanks for the most part have higher base damages on skills, but terrible scaling. This is to allow them to lane, and have relevant damage as the game goes on, but keeps them from wanting to build damage. Their damage is also attached to their skills. They have to use them to deal damage, possibly multiple rotations, and stay near you for the time between CDs. This is how they are meant to be. Now tanks in the early game go straight into defensive stats for items. Tabis, specters cowl, bramble vest, bambis cinder, etc. IMO resists early over health. Because the quicker you get your resists to 100, the further you reduce your opponents early damage. So at 15 minutes when that tank comes to fight you with 130 armor and 2000 health, your damage is greatly reduced against them, but you sitting on 38 MR take about 15-20% more damage from their whole rotation of skills.
It is a similar case for lethality users. Lethality is garbage vs tanks, as it directly reduces armor (think subtraction) 120-32=88 armor, so the tank still reduces all physical damage taken by about 45%. But against a fellow ADC or assassin, or a mage, lethality is great because none of these classes tend to build any armor at all, let alone early. So at level 9, lets say that Jhin has Youmuu's and Serated Dirk. That is 32 lethality, or 25.6 armor pen at level 9 (lethality scales per level). Now Jhin is attacking an Ezreal (who is incredibly popular atm) who has 54.6 armor at level 9. So against Jhin, Ezreal only has 29 armor. So he only reduces Jhins damage by 20%. All of Jhins damage.
Now if Ezreal purchased an early Tabis, he increases his armor by 30. So at level 9, ezreal would have 84.6 armor, 59 against Jhin's damage. So he would reduce Jhins damage by 35%. That is a 15% decrease in Jhins overall damage. If ezreal buys another cloth armor for only 300g, (15 armor) and sit on it for GA or sells it later, he reduces Jhins damage by about 42%. A further 7% than just having tabis.
TL-DR: While damage overall is too high atm, tanks will deal "high" damage to targets who refuse to buy MR, and lethality users deal "tons" of damage to targets who don't build armor. If against a lethality user, buying 45 armor for 1400g effectively counters their entire itemization, if against a mage or a tank, buying a negatron cloak early will reduce their damage, especially since tanks don't build damage to counter your purchase. Remember, damage wins you games, but resists and defensive play get you out of lane