\$\begingroup\$

I'm not initmately familiar with LoL or Dota, and a definitive or comprehensive answer will be hard to come by in any case, but allow me to share some mechanisms I have observed.

Choice

From a naive viewpoint, the more unique options you offer to players, the greater the risk that one of them will turn out overpowered and/or break some core mechanic. In Dota/LOL, the number of playable characters, each with their own unique abilities, is quite high, so if only one of them turned out too powerful, that will shape the metagame. To make matters worse, many players tend to more or less blindly latch on to whatever strategy's deemed the most powerful, exaggerating the imbalance.

A common approach to mitigating this is to introduce specialization via classes or roles (e.g. tank, spank, healer in most MMOs). The idea is that if different options compete in different categories, there has to be at least some diversity (e.g. at minimium, one optimal tank, damage dealer and healer instead of just one optimal character).

In practice, this approach carries the danger of pigeonholing both individual options (a damage dealer might now be measured only by their dps, making it easier to find the "best" choice) and overall composition (a party needs to have a tank and a healer).

The above is a pretty typical attempt to "pre-empt" a specific metagame by, effectively, codifying a different one, thus limiting (viable) choice. This may well be intentional, as it can make a game more accessible and easier to balance towards the chosen playstyle (Blizzard likes to do this a lot), but whether desired or not, it will lead to a "stale" meta that swings heavily with balance patches. LoL would probably fall into that category.

On counters

I would argue that where an option falls on the "hard vs soft counter" spectrum matters far less than how options are chosen and what mechanisms a player has to react to an opponent's choice.

Consider Starcraft 2, for example. As a rule of thumb, you have to pick a build before even scouting the enemy. Once you know what they're up to, you can of course react, but that takes time. Philipp has outlined that mind game aspect pretty well.

Say you went for Roaches and your opponent went Phoenix. That's (at reasonable numbers) a hard counter, the Roaches can't ever shoot up. However, it doesn't mean you lost the game, you're just at a disadvantage until you transition into something else, say, Ling/Bane/Hydra.

Now, in LoL/Dota, you can't switch champions mid game, but you can pick items and change up your tactics to adapt to a limited degree. The relative influence of these mid-match decisions compared to your initial champion pick determines the "wiggle room" you have for making a strategy viable, affecting the range and diversity of viable strategies.

Synergy and interaction

As a rule of thumb, options that compete, but do not (meaningfully) interact lead to one superior option emerging and the rest being phased out.

Spellcasters in D&D 3.5 had, among many other tricks, two very similar methods of defeating a monster: reducing their hitpoints to zero, or reducing one of their ability scores (strength, intelligence etc.) to zero. The latter turned out to be slightly easier, so that's what character-optimizing players would pick.

In this case, changing the numbers will usually just lead to a few days of chaos until players have determined the best option again.

In the interest of a diverse and malleable metagame, options need to interact. Whether this is in the form of counters ("rock-paper-scissors"), synergies or something else entirely, the idea is that the presence of one element changes the strategic role or value of another, shifting in turn the value of the inciting element (the principle behind "perfect imbalance").

Again, there is a risk of severely limiting your metagame if you don't provide a wide range of interactions. If fire damage is unusually common or powerful, and Ice Armor is the only counter to that, guess which spell everyone will pack?

Now, if I were to hazard a guess, based on PSquall's description, it sounds like LoL with its rigid role and lane structure might have hit this nail as well.