Yeah, that's what I said. The real issue is that Luigi doesn't really have any great attacks to slide into. Though it could still help him control positioning, space, weave, helping him to run his aerial mixups and get grabs.
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Luco
Mobility is so important I don't even know where to start. It's basically gives you a deep well of options. Delaying timings, exposing openings, repositioning, and many many other fundamental strategies expand as a function of mobility.
Developers never correctly estimate the strength of fast characters. If they did, slow characters would seem horrendously OP when the game is new. That never happened (except maybe for Bowser?), so guess what: the fast characters that seem balanced against slow, heavy hitters are just going to get stronger, while slow characters will still have the same old stuff. When people get comfortable with speed, and push the limits of it, slow characters benefit less and fall behind.
All the potential from increased mastery of the game is in character control, and mobile characters can do it the best. We all should know that positioning determines who is at advantage, and who is good at changing position? It doesn't take calculus to figure out that it's the characters with the velocity. Just changing positions introduces whole new situations to consider, and its this control that ends up controlling the flow of matches.
High speed means one may force more situations that can't be reacted to, which provides a wider threat range. True "range" isn't the distance from starting hurtbox position to ending hitbox position, it's the distance from where the opponent must commit or be hit, or when you can attempt a hit without committing. That's why Falcon has more range than Ike, a way.
Changing situations and forcing reactions gives one a massive psychological advantage. When you change positions, you change the situation, and you're aware of what's going to change before the opponent. This means it's on the opponent to react and adapt to something that you're already ready and aware of. That's the advantage of advancing your own strategy, or "taking the initiative." The faster character can almost always take the initiative more easily, advance their strategy, change the situation, because they can get the positions they want sooner than a slow character.
Speed suffocates thought. The faster you force your opponent to react, the less time they have to think and make correct decisions. No, thought and analysis doesn't occur at light speed. Brains are physical objects are limited by physical laws. When we don't have time to think, our subconscious minds take over, whose responses are a result of training, conditioning, not your overly long posts on Smashboards. We may be able to train to focus on the best things possible in order to improve and adapt reactions, but focus, too, is limited (multi-tasking is a myth). Who messes with that the most? The characters with the options, and the mobile characters.
Another side effect of good mobility is the ability to choose your battles. Risk/reward not looking good? Just run away until it's better. And those psychological factors I just mentioned? ****s all over contrived models of risk/reward that don't take them into account, and speed/control makes them more abusable. If someone's knee-jerk reaction is to shield, a grab at a certain timing when you've forced their knee to jerk will hit them 100% of the time, no risk. Good luck adapting something that you do without thinking. "Is he gonna do what I think he's gonna do or does he know that's what I'm thinking and he's gonna do something else, OR does he
know that I'm anticipating him knowing that I'm predicting what he will do ... " -> this line of thought isn't happening in the space of a half second.
Mobility-- and control of that mobility -- is king, and only gross differences in apparent risk/reward can make up for it. Heavy characters have to seem oppressively strong, or they must have some alternate kind of mobility or ability to take the initiative, to be threats. Or, general mobility in the game has to suck (i.e. overly committal). Or, the game has to thoroughly weaken the advantage of having the initiative (i.e. if the game rewards the defender/reacter too much, or that the slow character has good defense).
Most of this applies to all real-time games.