Short dota2 to LoL guide (longer version here):
Since you have to grind or pay to get all champions in LoL, the champions are divided into five roles, so that new players with a small champion pool will only be at a small disadvantage. The five roles are:
adc - carries are variations of drow/sniper. So high dps ranged glass cannons. There are no melee carries, or tanky carries. Their damage scales directly with items, so they have an optimal build. They go bot
support - supports are variations on abadon, earth shaker and omninight, assuming you remove their ability to do damage. They do healing / cc, and spam wards. They babysit the adc, and roam a bit, if they roam at all. They go bot
mid - mids are variations of lina/qop/puck/skywrath. Nukers. Unlike dota their damage scales directly with items, so they have an optimal build.
jungler - the level 1 jungler of dota mixed with the roaming support. It's a mandatory role. those are variations of mirana / axe, if you remove mirana's ult. They farm the jungle and then gank lanes. They are resposible for killing Baron Nashor (similar to Roshan) and the Dragon (gives a buff). There are also two neutrals that give a red buff (weak oov), and blue buff (mana regen+cd reduction).
top – tanks mostly. While tanky heroes exist in dota, tanks don't really exist above 2k mmr. Think axe or bristleback. You should watch someone play them. Their tankiness scales directly with items, so they have an optimal build to counter one of the two damage types in the game.
As you might have noticed, unlike dota, the roles and lanes are one and the same. If you pick a role, you usually pick the lane as well.
In LoL there is no denying, stacking, smokes, tp scrolls, RNG, buybacks, micro, custom games, cheats in lobbies, voice chat, replays, you can't check other champions abilities in game. Games are not safe to leave after a player abandons. There is barely any mana management, global movement, teleports, abilities that impact allies more than a buff, less comebacks.
There is a lot more focus on mechanical outplay, fast reflexes, skillshots, low spell cd, snowballing. There are shorter games, region lock, unchangeable names, weaker spells, better after game stats, you can surrender.
There are masteries, runes, and rune pages you need to grind. Runes give you stats, so if you don't have them you're at a disadvantage. Don't go ranked if you haven't grinded them yet. Only buy the most expensive ones (tier 3), although tier 1 are fine for normals.
There are summoner spells that you start the game with - like flash (short ranged blink on a 5 minute cd), teleport (tp on a 5 min cd), or smite (a pure damage nuke to neutrals for the jungler), and others.
There is no courier. Instead you can tp to the base for free.
Things you should read about when you have time, but there's no reason for me to detail here: dragon buff, tower agro, rosh buff, summoners, masteries, runes, damage types.
Other things:
Watch two or three games of pros in soloqueue - games are somewhat repetitive (same lanes, same roles, same pace, same builds) so you can learn it really fast. It's much more important in lol since you don't have access to 90% of champions, no replays, no custom lobbies, no in game guides, can't practice flash spots, and so on.
More than any other role, junglers are dependent upon their runes, and you won't be able to jungle with them till you get the runes. Every jungler has a different path to farm the jungle as fast as possible. Find a guide on each one to learn the optimal way to do it.
In LoL Vision is effectively unlimited. Abuse it, especially as the support. Imagine ganking a team when they have 10 wards on the map, smokes don't exist, and only 2/126 champions have invisibility – not gonna happen. This allows for more lengthy safe farming.
kiting means the same thing in both games, but it's done very differently. In dota you do it with items, abilities, and teamwork. In LoL you do it like drow does it – right click on enemy, use the cd on your auto-attack to get far from enemy, right click on enemy.
In general item builds are accurate, so stick to them if you care about winning. Everything works in soloqueue, but if you care about winning check Google. Pros use the same build 100 games in a row for a reason. Since the game lacks actives and item counters, there are mathematically optimal builds. Stick to the guides if you want to win, no reason to try new things.
Builds from the TI equivalent illustrate how itemization is done on the highest levels of play – same items every time. You can follow guides to the letter, it's perfectly fine.
Since you come from dota you might wonder how you can compete when most of the champions are locked, and it takes 68.17 weeks to of playing 10 games a day to unlock all champions, runes not included. Well:
Champions in the same role are often very similar to one another, so many of them are better versions of each other (quote from riot:"when two champs play the same way, you just end up picking the one that does the same things better. The result is the same folks played in every game (unless that power balance shifts), and each game of League feeling similar."). Only buy the better versions.
If you're not sure which are the strongest after reading tier lists, check what the pros do in the LCS. They pick the same champions over and over again every game, so you can easily tell which ones are the best.
In general, many champions in the same role do the same thing, although they might be drastically different in terms of mechanics (fork and chopsticks, Graves and Lucian). This is unlike dota where heroes might be similar mechanically but do completely different things (knife and handsaw, wk and sven). This means that in the draft you have to worry about picking one of each role, instead of worrying about synergy / counters.
Don't worry if you don't have time to keep up with the meta. For the past 4-5 years the dominant meta has been 1-1-2 + jungler, with Riot switching the optimal builds / strongest champions in each role. Just check the guides, chill, and have fun.
In most cases, champions can only be effectively played in one role. This is so extreme that when a top was played as support, Riot advertised it to the players in the client. Playing a champion in the wrong lane / role will you flamed - which leads us to:
The community is bad, same as in dota. Find friends and play with them.
This last one is a fair warning. Some people don't like to play certain games due to the companies that make them – EA, Hi-Rez, Riot. In this case it's due to Riot's belief practicing in custom lobbies is bad, their enforcement of the meta, the way they shutdown a well made community client, their failure to provide replays, and so on. Some of it may be wrong. I don't really care – I play the game, not the company. But if you do care you might want to read about Riot.
Each game focuses on different things, and offers a different experience, so I suggest you try LoL for yourself and see if you like it. It's free it's really fun, and you can always play both.
The effort needed to learn LoL might seem daunting, but compared to dota it is easier to learn. In the top bracket, a 1/3 of the champion pool is picked in less than 3% of games. Darius hasn't seen competitive play since May 2013. This means that from the mountain of knowledge necessary to play the game well, a huge part isn't terribly important and can be safely ignored for now. I know the game looks big and scary, but you already learned one arts, you can do it again. I believe in you.
Champions unique to LoL you should try: nasus, singed, oriana, syndra, udyr, lee sin, riven.
Heroes unique to dota you won't find in LoL: melee carries, tanky carries, niche picks (brood, huskar), hard counters to heroes, heroes countered by items, heroes who drastically change the way you play the game (techies, NP), vision altering heroes (beastmaster, aghs kotl / tree / NS, BH), micro heroes, supports who do damage (aa, cm, es), supports that can transition to carries (silencer, naga).
I made a shoddy chart to answer the age-old question of what champion should I pick that is similar to my favorite hero. The chart is obviously not 100% accurate, but it's a try, and I'll update it. If your hero is marked, there's probably no reason to ask this question.
Earth shaker after you remove his damage loses what makes him a unique pick - he can't win 1v5 at the rosh pit or counter illusion heroes – but he can still support and provide a lot of cc along with a long range initiation like Leona.
Mirana without ult can't make her team invisible, but she does have a wave clearing nuke, long range cc skillshot, and a dash, so she has many champions similar to her.
GL HF!
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