I feel that in the League of Legends many champs are considered OP. Well for example, a "problem" according to the boards (i'm staying neutral on this one), Yasuo has double critical chance. Well then why not counter that with an item that lowers critical chance on you(possibly an aura)?
1st - Yasou is an assassin.
He will get QQed about within the forums no matter how terrible he is. As a general rule, you shouldn't believe forum goers over the power of an assassin.
2n - Targeting critical strike chance specifically would be horrible.
Only 2 champions actually innatly have critical strike chance:
Everyone else starts the game with 0% crit chance, requiring items to actually have any crit at all (or runes).
So, basically anti-crit is going to be useless early game and only useful end-game.
To make an anti-crit item offer 20% more EHP against 50% crit chance, you'd also need to reduce critical strike chance by 50%, seeing as 1/5th of 150 is 125.
Or vs. Frozen Heart's unilatteral 15% EHP boost, you'd need to reduce critical strike chance by 40%.
Which basically means if an anti-crit item was ever any good, every AD could just build something like
or 
Rendering the crit item 100% useless (again).
This is just one of many possible items that could easily balance the game plus adding more fun with your builds.
That wouldn't balance the game.
It would just make caster-esque marksman
vastly supiror to AA-heavy marksman.
For seasons 1, 2, and 3 - caster-marksman were already better, that's the entire logic behind Riot nerffing the %AS slow in Randuin's to allow Tristana to be a viable pick in the pro leagues.
It would also make assassins with AD/AP ratios:
(All assassins besides Yasou and Yi)
suprior to assassins heavily reliant on critical strike chance for their damage ratios
And Yi only really saw competative play when you could play him AP, and AD-only scalers like Zed are doing a lot better than Yasou currently.
You're not giving Riot a lot of credit if you're assuming Yasou is just OP and needs to be dealt with through a fairly specific countermeasure item.
Riot is actually aware of how much damage his QWER ratio does - it actually requires a fallow-up with a critical striking auto-attack to match the QWER burst of most bursty champions.
Also - Yasou's double crit-chance steroid isn't even that impressive vs. non-crit steroids. At it's maximum strength (+50% crit rather than +0% crit) Yasou will be doing 180 damage while another champion will being doing 150 damage (or 225 vs. 175). You only need to have an AA steroid that makes you do 20% more damage (without IE) or 28% more damage (With IE) to out-DPS Yasou's AAs.
Friggin' Janna's +50 AD steroid will do that for the majority of the game, and she gets max-power on that as soon as level 9.
Why can't we have more life steal/Spell vamp and armor/magic pen items? Or maybe even more defensive items.
LIFE STEAL
Life steal is fairly easy to answer.
Life steal isn't unique like spell-vamp at the moment, as the system Riot's generally used works perfectly well for it ATM.
Which is mainly AA damage grows exponentially as you keep layering new stats.
Start with 100 AD and just get 4 more AD items, and you go 100+100+100+100+100 = 500 DPS.
Get 100 AD, 150% AS, 50%+50 armor pen, and 80%x2.5 crit damage and you go 100 x 2 x 2 x 2 x 2 = ~1600 DPS (or the equivalent of 1600 DPS assuming your opponents have at least 100 armor, average @18 is 75).
Riot's primarily kept life-steal in check by keeping it off crit and armor penetration items, so going more than 1 life steal item can often result in about half the damage you could get otherwise, whether your a caster (Armor pen) or AA heavy (Crit).
However, if Riot expanded the range of things life steal items could do, you could easily run into a situation where the trade-off isn't 500 DPS with 500 of that as life steal vs. 20% of 1600 DPS as life steal (320), it would be more like 1600 damage with 320 vs. 1400 with 1000. Which could cause problems.
SPELL VAMP
We can't have spell vamp because champions like Vladimir broke it so horrendously it needed to become a unique passive.
While AP does have secondary multipliers
(
and up until recently DFG)
There isn't anywhere close to the same opportunity cost in just stacking AP on a wide range of champions. 1 AD item can easily double your DPS, 1 AP item? you're usually looking at more a 33% increase in DPS over 100%.
A lot of AP damage is in bases alone (~40 per level rather than ~3.5 for AA damage), which opens up AP for more abuse in itemization than AD. This can also generally be applied to why there's only ~4 spell-hit effect items rather than the 20+ on-hit effect items available to AA champions.
As it currently stands
PENETRATION
100 armor offers 50% damage reduction, so 100 armor penetration will double damage inflicted.
Base armor is ~75 at level 18 on average, with a low of ~60 and high of ~90. As armor penetration currently stands, it's a pretty even deal against someone with 90 armor. It does enough to be a solid damage multiplier on AD Casters without going so overboard it actually adds more than double damage against anyone building armor.
So it's kind of at a critical mass of sorts - add more armor penetration and you could easily invalidate armor as a stat, or at the very least make it so niche it feels terrible to grab.
Granted - a typical game only really sees 3 major items and going %armor pen with Cleaver, Whisper, and something else has it's down-sides and still opens up such a build from being exploited early/mid by armor, but the general sentiment with the current armor-pen levels is armor already feels kind of bad to get with the amount of armor pen avalible.
Magic pen could possibly go a little further, though due to a big part of magic damage being in ability ranks, ability damage having lower multipliers, MR items not having the same high-value chunks as armor, and it not feeling as terrible to counter-build as phys damage, it's probably at a pretty solid spot ATM.
heck, I'm kinda bored with seeing the same build of some champs.
That doesn't really change with more items. People will just figure out another ~3 core items they should always have. And then Riot will balance said champion over their best build.
Granted - it's annoying seeing every champion of a given archetype grab the same thing over and over again. In season 1, IE + PD + LW was essentially the only ADC build (unless you were Corki because tons of damage). At least now there's options.
What I'm trying to say here is that if we had more items in the game with more passives, more actives, and more stats, then counter-building would be easier, and promote more item diversity with more builds.
I'm not sure if more stats would encourage counter-building so much as snowballing.
With AA power, grabbing a mix of AD, AS, Crit, and armor pen is the difference of exponential growth over addition. Most stats in LoL actually stack this way - technically speaking defenses scale with offense through merit of living longer resulting in a longer time to deal damage, and killing things faster resulting in taking less damage.
Add a new halfway decent AA modifyer, ability modifier, or defensive modifier and you could easily cause every damage-dealer to deal 50% more damage and every tank to survive 50% longer.
Which is sort of a problem when you consider the team that's ahead is going to be that much further ahead, also when considering tanks are going to be dealing even less damage and squishies are going to get obliterated that much faster.
Also, the gap in defense/offense between champions in LoL is already pretty intense compared to a wide range of other games.
Finally - there's a 100% chance you'd make the game more confusing/add a greater burdern of knowledge on all players by adding more items. 1 or 2 new items is certainly exciting, but for pretty much any art: "keep it simple stupid" is a pretty solid motto.
