Summoner's Rift isn't all roses
Before we even talk about Dominion, let's review Classic first. Summoner's Rift is not a bed of roses. It's not a magical, perfectly balanced game. It has over 90 playable characters, and there is simply no way that all of them are going to be equal. Even when we divide the champions into roles, there are a handful of "best" picks for each role. If we look at pro matches, they heavily support our theories -- there is a pool of 20 or so champions that are picked or banned very regularly, and over half the cast is rarely (if ever) used. In casual play, even pros pick these characters occasionally, but aside from Moscow Five, big teams rarely pick champions perceived as weak.
People who create tier lists generally group champions up into four major tiers. The first tier is often the top champions that are stable and versatile, while the second tier is composed of strong champions that have hard counters or rely on a gimmick or specific composition. The third and fourth tiers are generally weaker champions, with the third tier considered "somewhat weak" and the fourth considered mostly unplayable. Although many people use language in their tier lists that tries to avoid offending people, a champion rated in Tier 4 is usually easily countered by many Tier 1 and 2 champions. A fifth tier, usually called the god tier, is reserved only for extremely imbalanced champions at the top level of play.
Summoner's Rift is fairly well-balanced in that it has no universally accepted "god tier" characters. Ryze is banned frequently in high-level play, and Sivir and Sona are frequently considered the best carry and support respectively, but it is rarely those specific characters that break games. Generally, the early game is mostly about matchups (your champion vs. your lane opponent's). Some champions have a lot of good matchups, but none of the Tier 1 or Tier 2 champions is uncounterable.
Champion balance in the mid and late game is mostly about composition; if your team is better at teamfighting, you will be at an advantage, and if your team is not, you will have to seek advantages in other ways. There are lots of ways to do that, though, and aggressive pushing or counter-jungling can make it very hard for the enemy team to assemble and take a single objective. Individual champions are rarely as overpowered as the interactions produced by combining powerful abilities. Strong AoE crowd controls can make it easy to land powerful AoE nukes, for instance.
The first major problem with Summoner's Rift is that Tier 1 is relatively small. Datamining high-level play shows us a pool of a little over 20 characters that get picked a lot. This is not a huge balance problem because even if LoL had only those 20 characters, it'd still have a lot of depth. It is a problem because players want to see lots of champion diversity. If someone is a Katarina fan, he might be disappointed if the Sinister Blade never gets used. Also, finding out that a character you bought is actually very weak is not fun at all. It feels like a big waste of IP (or real money).
I found this out the hard way with pre-buff Mordekaiser; he was really weak, and I could only really play him in bot matches. Since he was buffed, he got a few good matchups and is probably Tier 3 material, but I still can't play him most of the time. He's a counterpick for a few specific matchups.
The other major problem is that due to the strict roles that are enforced in SR, some champions just aren't useful. A champion who is poor in the lane and is a slow jungler is very hard to play in SR. Poppy is a great example of this. She is difficult to lane with, she is easily harassed, and she has no real chance of killing a cautious enemy. Additionally, Poppy is a poor jungler with no sustainability. She's a great ganker, but that's all she can do, and if the enemy team responds by stealing her jungle creeps, she can get behind very quickly. Poppy has no real place in SR, sadly. There are a few other champions that have similar problems, but Poppy is one of the most glaring examples.
SR's biggest balance issue is that over half the cast is pretty bad and rarely used, and some champions are so bad that they have no real purpose at all except to slum in the bottom tiers.
Dominion isn't balanced because of Falcon... erm, Rammus
Unlike Summoner's Rift, Dominion's viable champion list is huge. Although I suspect it will shrink a little as the game gets more and more "figured out," I can list around 60 champions that have "overpowered advantages" on Dominion. I'm also of the firm belief that Tier 1 is much larger, probably composed of half that number. Although there is no pro scene to analyze, the high-level Dominion experts vary tremendously in their opinions. Some still believe that Akali is Tier 1 despite her nerfs. Some think Heimerdinger is practically god tier. Some think Gangplank is undisputably the best character. I happen to disagree with all of these things, but what these differences of opinion tell me is that the top level of play has a lot of diversity.
Remember when I said that the mid and late game in SR was mostly about composition? Well, aside from the bottom lane, individual matchups are nonexistent in Dominion. It's all about composition, and that means that the individual merit of a champion matters far less than what she can do with a team. Katarina is considered kind of weak, but the opportunity to pair her ultimate with Galio's happens as soon as both champions hit level 6, not after Katarina has suffered brutally at the hands of her lane opponent for 15 minutes. Single powerful champions still have a lot of weight; Ryze is still one of the best choices in Dominion, and Poppy -- a total wallflower in SR -- is a team-soloing terror in Dominion. Team composition just matters a lot more.
The largest criticism of balance for those in the know is the bottom lane. The bottom lane is most like the middle lane in SR and generally involves two champions fighting against each other in a mad push to take the others' bottom turret. It even has SR ganking mechanics; there's an ever-present threat of a jungler coming to gank you.
All this means is that individual champion and matchup matter a lot. In general, it is very hard for most of the cast to do anything against Heimerdinger in bottom lane. If your team didn't field a counter to him, he will ruin your team's day. As Dominion evolved, other champions, particularly Ryze and Yorick, rose up to claim the "unbeatable" title. However, these matchups have been analyzed more than anything else in Dominion, and we know that pretty much every champion has at least a few counters. (If you want a quick tip, though, Cassiopeia beats everyone except anti-caster assassins.)
Dominion holds up pretty well balance-wise in most respect. Most of the cast is very strong in some way (far more than SR), and virtually no one is unplayable. Most of the top characters have counters. And therein lies the problem.
Most of the top characters have counters. The problem with Dominion's balance is a very small handful of undisputably "god tier" characters. Virtually every expert Dominion summoner knows these champions. The problem is generally not that these characters win matchups or provide unbeatable synergy but that they provide ridiculous amounts of map control. It is virtually impossible to counter the ability to cross the map in 10 seconds.
The two characters I'm talking about are Rammus and Kassadin. Rammus' Powerball lets him accelerate to ludicrous speeds, letting him fly across the map like a blue hedgehog. Rammus can be anywhere at any time. If he is at his summoner platform, he can be at the top capture point in around 10 seconds. If he's in the middle of the map, he can reinforce anywhere he needs to be in a few moments. If he's at the top capture point, he can gank the bottom lane and be back at the top capture point in time to stop the enemy team from neuting it. Rammus is a powerful teamfighter, too; he has two crowd controls (one of them built into Powerball) on relatively short cooldowns. He excels in small skirmishes that are common on Dominion.
Kassadin can teleport over literally any obstacle on the Crystal Scar. He can Riftwalk from his summoner platform into the lane between his mid and bottom, instantly. With skilled play, Kassadin can use these shortcuts to quickly get across the map. He is not a teamfighter like Rammus; he's an assassin. When he arrives late to a fight, he can often immediately kill a single wounded enemy, and the threat of striking again often forces other weakened foes to flee. His burst combo also silences, keeping the enemies from using Flash to get away. Even if they do, Kassadin can just Riftwalk after them; Flash has a cooldown of over 200 seconds, while Riftwalk has a cooldown of six seconds or less.
In a stand-up fight, neither of these characters is overpowered. It's the issue of map control and movement that pushes them over the top. So naturally, Dominion is worse than SR, right? Having two god-tier champions makes Dominion that much worse? I don't think so. First, draft mode has bans, and these two champions are banned almost all the time. This can lead to some fun "chicken games" with bans. Second, while their mobility doesn't have direct counters, Rammus and Kassadin's actual fighting ability can be mitigated with the right characters or items.
I'm not actually sure if having two "absolutely the best ever" champions makes Dominion unsuitable for ranked, but I do know that SR's balance has been in a lot worse shape than Dominion's balance ever has been.
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