Disclaimer: The OP was originally written by a heavily biased player who’s fairly known for having a bizarre and erratic playstyle. I’m going to try to make it more “middle-ground” as people offer input later, but understand that this is just one perspective on a lot of these things.
Do you like stomping people out of the lane by taking advantage of superior micro? Do you like having (usually) the best selection of Crowd Control effects? Do you like swooping in like a hawk to punish a mistake, then have the ability (usually) to get away?
Then playing a spellcaster champion might be for you.
Spellcasters are generally hit-and-away heroes, that have a period of dominance ranging from early-to-mid game, and in a few rare cases, even into late-game. They’re not as lock-down as ranged carries, but they’re less reliant on meatshields, and tend to have less variance from game-to-game. Because they have such good utility, they’re also welcome on many teams, and their flexibility always affords multiple effective ways of building. They reward positioning, opportunism, flexibility, and game understanding. And most teams have the ability to carry two of them. They have some of the most unique skills in the game, leading to some bizarre sexy plays, or funky kills and get-aways. They can, however, also be the most useless pieces of dead weight in the game, can be heavily affected by a bad early game, and are generally relegated to support by the end of the game.
Welcome to the thread of assholes: we like making peoples’ lives miserable.
General Play Advice/Tips
Lane control and presence
Actual lane control depends heavily on your hero. As with all other heroes, you MUST know your damage output, your range, and the damage trade-off with an opponent versus your ability to fight attrition. You must also gauge your opponent's reaction time and decision-making. For example, I once laned Alistar against a Sona and Shen, and I recognized right off the bat that Sona was pussy and Shen was slow. I rammed Sona like a boss, chunking her down 30% hp at a time. Once the creep pushed I rammed her into a wall, follow-up stun, game over. If the Shen were better though, and if the Sona were better at gauging damage and trading hits with me, it wouldn't have been so one-sided. A simple example of a good spellcaster player versus a bad one. Moreso than any other hero archetype in the game spellcasters need to have strong lane presence. When we cast spells we don't incur creep aggro - this means we more than any else have the ability to punish people when strong. Do it.
This is more of a basic gameplay fundamental but you must learn to dance effectively in all matchups. Not only is this good for all matchups anyways, this is crucial for spellcasters who lack sustained damage to fight back against dps if you're caught.
Understanding burst vs sustained, and taking advantage of burst
How creep aggro works
Map control and presence
Knowing when you're strong
Targeting priority and roles in team fights
Taking advantage of CC, burst threat
Knowing when to switch offense defense
Esp. when you have certain heroes or summoner spells
Flash and Ghost
Almost every spellcaster will have one or both of these spells (until Flash is removed), hence why this is placed at the top. Different spellcasters/playstyles have different needs, though. For example, Flash on Annie is non-negotiable because she needs that instant close-gap (to take advantage of her instantaneous AoE stun. Compare this to Sona's AoE disable which has a significant delay on its effect, making Flash/Ult not as devastating) and re-positioning. In almost all non-similar champion situations, I'll repeat what I've often mentioned. Flash is great for crossing walls (often a life-saver, other times it simply prolongs the inevitable) and for dodging critical projectiles, but Ghost has significantly better distance covered, and its duration allows you to fire it and have its effect for a long time over the duration of a fight, while having better cooldown.
A lot of people will give you arguments for and against taking this. I think it's actually one of the best scaling summoner spells in the game, given that your primary targets (physical DPS) get stronger and stronger as the game goes on. Hence, blinding one of them for 2 seconds 50 minutes into the game is ridiculously good. But it's a tough sell for a lot of spellcasters because they need other things - re-positioning, added burst, more mobility, etc. You'll more often see this on specific support heroes, like Sona, or as a counter to a hero like Twitch.
For the most part this is a straight-forward summoner spell: it adds to your burst. It also has a "secondary" use for countering heal, particularly heal teams, or against certain champions (Vlad, Mundo).
Like Exhaust, you'll mostly see this on support heroes who don't need to re-position themselves as often, for instance, to ensure strikes at opponents (as you tend to babysit your own), or don't need to worry about laying down extra burst, or don't need to worry too much about survival, etc. It's an underrated spell, or rather it's not taken as often as you might want, but it's not a simple "it's good - take it" either because the other summoner spells are great and the likelihood of Clair tipping a game in your favor isn't that big. Like wards, it requires a team dedicated to taking advantage of the vision.
Certain spellcasters are heavily reliant on sustained mana usage to survive/control their lanes, like Kassadin, or are support heroes that also rely on a steady stream of mana like Janna and Taric. In both of the latter's cases you can make do without, but many players like taking Clarity as insurance, to prevent losing their lane to a "superior" lane control hero. Taric in particular had a stretch where it was almost 100% required to take Clarity so that you could hold your own against a Soraka lane. Be aware of the heroes on your team though - sometimes no matter how much you might want it individually, it just might not be worth taking Clarity when you're on Shen/Taric lane with Kennen solo top.
Particularly when you're solo top, Teleport is great to have for top-to-bottom mobility - this is a huge advantage when it comes to dragon and golem control, as well as, later in the game, bottom-to-top mobility for controlling Baron. It's also nice for some nasty teleport-onto-ward ganks when using certain insta-gib heroes, but in both cases this is reliant on good teamwork and warding and coordination.
Post-nerf most people swear by not using Cleanse, and it's definitely become more of a counterpick (ie. vs Ashe etc.) or a luxury spell. On certain playstyles/heroes where you're always in their face, having Cleanse can be the difference between life and death. Other people, particularly those confident in their reaction speed, simply rely on Flash/Ghost (for instance) to avoid projectile-based disables. This is going to be up to you and your judgment when it comes to how much you need Cleanse. There really is no easy answer.
Revive, Rally, and Heal
Niche or out-right bad: I can't think of any spellcasters that use this. You can kind of make an argument for Rally (70 AP at level 18) but...
Solely for jungling purposes - a handful of "casters" can jungle, like Nunu and Sion, but those things can and will be covered in their respective threads. For the most part, though, steer clear.
Extremely niche. Aside from a gimmick 5-fort team you're probably never going to use this.
Runes and common runesets
Doran’s Ring vs Doran’s Shield
This is a surprisingly difficult decision for many heroes: you want the mana regen, but the base mana regen for casters tends to be kinda okay, while the HP regen tends to be terrible. Buying a Meki Pendant to open is very discouraged because that lack of 130 HP can and will get you killed or zoned. This is actually the case for Sapphire Crystals as well.
Shield gives you a crucial bit of HP regen, which you will definitely feel against good opponents. You’ll have to play your hero and determine whether you need HP or mana more (take into account how fast you chew through mana) .
For many heroes this (Crystal + 2x heal) isn't recommended. The simple reason is that the lower base HP does affect some things. For example, good players will generally not allow you to beat them down in a one-sided exchange, especially knowing that you're a spellcaster whose ranged damage is limited by cooldown... and by casting range. To top things off, the majority of opponents you'll face will also pack some sort of burst. This is just the way the game is. This puts a premium on keeping your HP above a certain threshold, and also winning slugfests of attrition. To this end Sapphire Crystal is "not good."
There are exceptions though. For example if you’re an Annie player, you have lane control via your massive instantaneous burst. That means that you have less HP to burn through - juxtapose this to a hero like Tristana (ignore that she's not a spellcaster for a second). Good Tristana players whittle you down with active Explosive while trading blows with you. Once you hit about 200-300 HP you're at risk of being insta-killed by a Rocket + Explosive combo. That means if you have 800 HP he has to cut through 500-600 hp. On the other hand if you have 500 HP you might as well be half-dead. This is a bit of an exaggeration because you're only really down 130 but it's noticeable. On the flip side Annie's level 6 burst is some 500-600 damage (I forget), which lends itself to some tense HP jungling for the opponent... and is why she's a strong laner. Thus, with such a burst advantage, you can get a Sapphire and not have to worry about the disparity in HP buffer.
Just keeping this here for posterity - Meki Pendant is a terrible opening because mana regen is a weak stat, and you end up with the same opening EHP (via 2x pots) as a Sapphire Crystal.
Yes, this is actually a legitimate opening for a select few heroes: Jungle Fiddle comes to mind, and if you're confident in mana management, this can let you jump-start into something like a really fast Soulstealer.
The simple answer is,
damage = sorc boots
mobility = swiftness/mobility (based on hero/proximity to combat)
survival = treads
Feel comfortable swapping boots mid-game, I've had many games where I open with Sorc for the early/mid-game damage boost and then I swap to Treads as I start getting FF'd and I have Void dealing with MR. The swap from sorc to treads makes a HUGE difference.
You might be surprised at how not-good this item is, but it has its spots. If you need emergency MR, the Negatron cloak in the recipe can turn into Abyssal if you want MR but don’t want to invest into a pure defensive item (Banshee’s Veil/Quicksilver Sash). The –MR is not going to be too visible unless you’re Fiddle or Amumu, in my opinion, especially once you get Void Staff.
Pondering AP ratios, and what they mean