I do know that item sets have been buggy or odd for some people, though I have not had that issue. I have only created two or three, however, and do not really use them too much. Here is why:
Once you get playing a bit, you realize that there are a huge number of items, but the actual number of optimal or intelligent items is much smaller. This is for two reasons.
First, once you get into the strategy of the game, you discover that many items are simply not as good as others. In League of Legends, this is often expressed in terms of gold efficiency. In simplest terms, if item A gets you +10 foozles for 100 gold per foozle, while item B gets you +20 foozles for 50 gold per foozle, the savvy players of the game will recognize this and always lean toward item B. This information then eventually percolates down to the unsavvy players like me, who slavishly copy those choices. :)
There are a lot of items like this, where except in rare cases (which are typically rare enough that casual players like you and me don’t need to worry about them), the item is outclassed by a better item that plays a similar role and thus you don’t need to remember the outclassed item or consider it as a choice.
So through this mechanism, the set of items you need to worry about shrinks.
Second, you start to associate item “sets” with three or four overall types of characters and roles. Though a bit clumsy, I would broadly define these into (i) characters that are offensive Attack Damage (you will constantly see this shortened to “AD”) characters, (ii) characters that are offensive Ability Power (you will constantly see this shortened to “AP”) characters, (iii) characters that are bruiser/tanky type defensive characters (these are also technically AD or AP, but generally focus more on defensive items as opposed to offensive items); and (iv) characters who are “support” types of characters.
With characters who are offensive AD type characters (the classic example of this is the ADC, or “marksman” as League is trying to get it renamed, that plays in the bottom lane along side a “support” character), you tend to buy from a small subset of items that have any or all of the following: bonuses to your AD, bonus chance to critical hit, bonuses to attack speed, and reduction to an opponent’s armor (because their armor blocks your Attack Damage). So when you play these characters for awhile, you get used to the familiar items you buy that meet these task. For example, the classic ADC/Marksman build typically involves buying some or most of the following in some order: a Bloodthirster, Blade of the Ruined King, Infinity Edge, Phantom Dancer, Static Shiv, Berserker’s Greaves (boots that increase your attack speed), Last Whisper, Black Cleaver. All of these do one of the four main things mentioned above for offensively oriented AD characters (increase AD, increase attack speed, increase your chance to critically hit, or reduce opponents’ armor), and have other side effects dependent on the items (the Berserker’s Greaves, for example, increase your movement speed as well as your attack speed).
With characters who are offensive AP type characters (AP mage types like Annie or Cassiopeia ), you tend to buy from a small subset of items that increase your AP, reduce your cooldowns on your spells, and reduce your opponent’s magic resistance (because much like armor prevents damage from AD attacks and AD based skills, magic resistance prevents damage from AP based skills). Certain AP type characters are often very mana hungry, and may also buy an item or two that increase their mana pool or mana regeneration rate. Like with offensive AD characters, with offensive AP characters, again, while there is a lot of choice it generally boils down to a number of “usual suspects” that most characters buy. These are things like Rabodan’s Deathcap, Liandry’s Torment, Zhonya’s Hourglass, Deathfire Grasp, Abyssal Scepter, and Rylai’s Crystal Scepter. Certain items provide AP and increased mana (overall pool or regeneration rate) like Athene’s Chalice, for those who have greater mana needs.
Defensive/Tanky characters build primarily armor, magic resistance, and health items, to soak up damage and be more survivable. Whether you lean toward armor or magic resist depends primarily on two things: (i) the mix of the other team (if they have more AD characters who do damage you lean to more armor, if they have more AP characters who do damage you lean to magic resist); and (ii) in game how the opponents are doing - if one guy on the other team is getting really fed and strong with a lot of kills against your team, you tend to buy armor/magic resist to counter whatever he is (armor if he is an AD character, magic resist if he is an AP character). So items like this would include Sunfire Cape, Spirit Visage, Randuin’s Omen, Frozen Heart, and Warmog’s Armor.
Supports are a little strange. They tend to start by buying items that generate gold (primarily Philosopher’s Stone and Kage’s Pick). That is because they do not last hit minions or tend to get many kills themselves, so they don’t generate a lot of gold through ordinary in-game activities. Supports also have the primary role of buying and placing wards for map vision, so they then also end up usually buying a Sightstone (which they upgrade to a Ruby Sightstone), which allows you to place “free” wards. After that, they continue buying additional wards, Oracles potions to see and kill the enemy’s wards, and then with any money left over, a few items just like the above mentioned characters do (e.g. Leona, who is a tanky support, would probably buy some armor or magic resist, whereas an AP support like Lux might buy an AP item or two).
As a “subset,” there is also the Jungle character. They primarily differ in that they usually start with a jungle item (like Spirit of the Ancient Golem) to help them clear the jungle camps faster and more safely, but from there generally build per the above character types, depending on what they are (Jarvan, for example, is an AD type, and would build AD items and armor and magic resist, Maokai is an AP type (tanky), and would build AP items and armor and magic resist).
So once you play for a little bit, you quickly pick up the general “good” items for the type of character you are playing. For example, if I were playing just about any ADC/marksman character as a beginner, I wouldn’t feel very bad about picking up a Bloodthirster, Phantom Dancer, Bersker’s Greaves Infinity Edge, Last Whisper, and then maybe at the end some type of defensive item to give myself more survivability. Could I optimize that for a particular character for various in-depth reasons (e.g. Vayne is an ADC who because of her high base damage probably does better with more additional attack speed early, so I might buy a Blade of the Ruined King instead of the Bloodthirster)? Sure. But that’s really fine tuning the last 10%; as a casual/beginner, you are fine so long as you are picking from the right overall category and from those common items that are most gold efficient.
As a final note (and maybe this should have come earlier), the items I mentioned above are all “end stage” items. There are a bunch of sub-items (e.g. Blasting Wand) that go into making them. But if you remember the end stage items it is a lot easier, because you can shoot for them and the game tells you (when you click on the end stage items) the components you need to get to the end stage item. So you don’t have decide between and remember 20 individual items that go into the final “end stage” items you want; just figure out what end stage item you want and buy the components based on that.
So let’s say I’m playing Miss Fortune (one of the aforementioned ADC/marksman). I decide I’m going to get that Bloodthirster as my first item. Only it costs a ton of gold (3200g), and when I go back to base for the first time, I only have 1000g. The game makes it easy on me, because I can still just click on Bloodthirster, and it will tell me that it needs a B.F. Sword (1550 gold), and a Vampiric Scepter (800 gold). The Vampiric Scepter itself will be shown as being made up of a Long Sword (400 gold) plus playing another 400 gold to upgrade to the Vampiric Scepter.
So I only have 1000 gold, but knowing that I want my first “end stage” item to be the Bloodthirster, I simply click on Bloodthirster. I see that I can’t afford the B.F. Sword portion of it yet (because it costs 1500 gold). I can afford the Vampiric Scepter portion, so I buy that. Then, the next time I go back, I can see whether I can afford either the entire rest of the Blood Thirster, or maybe I can only afford the B.F. Sword. Or maybe I can’t afford either, so I can also then decide to start buying components toward another end stage item, and I’ll finish up the Bloodthirster later. To do that, I only really have to know about the Bloodthirster - I do not have to individually remember and think about the Vampiric Scepter, Long Sword, and B.F. Sword, because the Bloodthirster will remind me those are the components when I click on the Bloodthirster.
By having that overall, high level understanding of how the characters work, it really boils down your choices and makes it a lot easier to just have a rough mental pattern of what to buy for each of the 3-4 character types without having to worry about lists of specific items for each specific character.
