If you don't play a lot of Dominion or Twisted Treeline, you might not realize that Bloodthirster and Infinity Edge are hard to buy when fights can erupt faster than ultimate skills can recharge. Expensive items like a Needlessly Large Rod or BF Sword are hard to justify when you can get some interim item that provides more balanced stats and will help win the fights you're fighting now.
Playing other game modes also gives you a broader look at League of Legends. You don't see the value of certain stats -- particularly HP -- until you realize that an extra 200-500 HP can cause a huge swing in the course of an engagement.
A world where health potions aren't guaranteed
Dominion and TT are crazy sometimes. In Dominion, you are fighting or jockeying for a fight pretty much all the time. The post-fight recovery period in Dominion is incredibly short; generally the time taken to recall to base, buy, and spend about 15-20 seconds walking back is all it lasts. Encounters generally happen in one of two ways; either one team ambushes the other in the jungle or the two teams throw harassment at each other over a capture point until one team's initiator sees an opening and decides to take it.
In that environment, buying a health potion doesn't guarantee that you'll get even half of the HP from it. In fact, a lot of the time a fight will erupt immediately as you arrive to the battle. It's easy to forget that in Dominion, buying pots doesn't win you the long attrition battle that is SR laning. Also, because basing doesn't take away all of your income in Dominion, there's no reason not to do it if it'd be risky to stick around otherwise.
Because pots are kind of limited in use and fights are very frequent, base trips (and deaths) become more frequent, buying is more frequent and it becomes more important to buy something that helps right now instead of saving your gold for a rainy day. If there's a lot of burst magic damage on the enemy team and you have 800 gold, buying that Negatron Cloak right now will make a huge impact on the next fight. It's probably more worth it to do that and try to work it into some other item like an Odyn's Veil or Quicksilver item than to save up for a BFS.
Compare that to laning in SR. If your opponent deals burst magic damage, you should probably play safe and farm defensively, avoiding conflict unless she burns her cooldowns. Having Akali or Diana as your lane opponent doesn't automatically mean you need to buy MR or health since you can avoid getting hit by them and still accomplish your objective: farming. If your goal is to fight her, you still want damage because she can retreat to her big scary tower. You want to kill her quickly.
Twisted Treeline is similar to SR at first glance because it has lanes, minions, towers, and a jungle. There's a lot of PvE-type gameplay, and farming is an option. However, the similarities really end there.
Laning in TT is a lot different because the lanes are extremely long. I italicized "extremely" there, but I would put it in big 48-point bold font if I could. TT lanes are massive. Even being in the middle of the lane puts you a really, really long way away from your first tower. If you don't have sight in your jungle approach all the time, you are in deep trouble. There aren't any wards in TT. That means you're pretty much always in trouble unless your jungler is clearing his wolf spawn or you can see all three of the enemy players.
In fact, hyper-aggro jungling is pretty much the name of the game in TT. There's an altar in each jungle, and both teams want both altars. There's jungle creeps to steal, and they're worth a lot. If you can see both enemy laners, you can often coordinate an invasion to kill the enemy jungler and steal his altar. Basically everything in TT revolves around jungle roaming because there are no wards and moving where the enemy can't see creates fear and paranoia in their minds.
The natural evolution of hyper-aggro jungle play is -- you guessed it -- fighting all the time. The jungler takes the worst of it, but even the laners need to get out of lane frequently to respond to predicted ambushes or to create opportunities. Fights with four to five combatants (there are only six in TT) happen before level 6 all the time. Fights with four to five combatants almost never happen in SR before a tower goes down except as part of a very early, coordinated level 1 invasion. Most of the time, a bottom lane ADC will be level 8 or 9 with thousands of gold worth of items before she sees the enemy top lane player on the same screen as herself. In TT, invasions can start as early as the first altar unlock.
If you're fighting frequently, you might still be tempted to go for offense. If you eliminate the enemy early, why buy defense?
At low levels, a chunk of bonus HP is huge. A level 2 spell nuke deals 100-120 base damage. A Ruby Crystal is 180 HP, considerably more than one hit. 20 AP (much more than 500g worth of AP) would add somewhere between 12-14 extra damage on that hit instead. If two Annies trade nukes at level 3, the one with the extra health is going to win. In the late game, a fairly tanky mage with items like RoA and Rylai's will probably withstand the full burst of a Deathcap + DFG mage, and the RoA/Rylai's mage still has enough AP to kill a glass cannon mage in one combo (it's character-dependent, obviously).
For autoattackers, it's a little less clean-cut. A physical attacker probably won't be doing most of his damage with base numbers so getting +10 AD is a much larger percentage of damage than +20 AP. However, AD is incredibly expensive compared to AP. The short answer is that if you deal spell damage with base ability numbers (even if they scale to AD), building survivability items or penetration items helps you out a lot more because your base numbers do a lot of the work. If you rely on autoattacks, you need to build mostly pure attack because most of your damage is from items.
If you fight all the time, though, building pure attack generates somewhat weak benefits. Even though you will eventually multiply your damage through attack speed and crit chance, HP and armor (or MR) multiply on themselves too and you can't contribute as much as someone who has sweet base numbers and a lot of survivability. This generally means that, outside of SR, ADCs are kind of weak. If you're fighting all the time, having a liability for 60% of the game is not an ideal scenario even if that liability eventually wins.
The point that I alluded to last week though is that in general, the recent buffing of defensive items has caused a big shift toward building defense. I'm honestly not sure how long the hyper-squishy glass cannon-type builds will stick around when building HP and penetration is so much more efficient. ADCs have already gotten to the point that playing one without a strong escape is now a total mistake. Mid mage picks have evolved to be either more support-oriented or survivable than before or can simply fight after dying (Karthus will always be strong). TT and Dom players noticed these trends at the start of S3, but they're starting to creep into SR as well.
"Bruiser meta" is one of those terms that keeps getting tossed around, and more and more picks in SR (at least in LCS) have reflected this sort of trend. Defense has always been strong, but there's a reason why building mages in TT with basically no damage other than a Void Staff and a Liandry works. It's because when you can tank the enemy's upfront burst to the face and laugh, it doesn't matter that yours does half the damage. You have three or more times the effective HP. It's pretty obvious who wins that encounter.
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