Hopefully I can articulate well enough that this may be the "go-to" post if you ever want to suckerpunch someone in the shame gland for talking out of their *****.
Let's start off with the primary objective in most fighters. Each opponent is given a health bar, and you have to find a way to deplete your opponent's before they deplete yours. Once you hit the opponent enough, they die and you win. Pretty vanilla. Now let's look at variables that can affect this or make it more interesting. There are none. It comes down to simply putting out more force. Due to restrictions such as being grounded, and not moving very fast, this removes a major could-be facet from the fray of most fighting games. It comes down to working in a little box in close quarters. A Judo match would be a good analogy. This brings up a few more points.
Stages. The stages in most fighters change almost none, with the exception of minor stage hazards. In Soul Calibur we have ring-outs, which are the gayest thing God ever put on this earth. In DoA we have the occasional electric fence that you can wall combo someone on to death with Jan Lee. Again, nothing too special; nothing that forces the players to adapt and cope. Lastly, we take a look at unique characters. The one thing that arguably ties fightings games together in the most meaningful way, is how the characters operate. The only thing separating most of these games though, is not how unique the characters are, rather, what system the characters fight with.
A direct-counter system like Soul Calibur. A pressure system like Street Fighter. A combo-heavy system like Mortal Kombat. A juggle system like Tekken. But what is always kept the same, is the similarities that tie each character together. Every character has a basic set of moves that don't deviate from one another: punch and kick variations (such as high/low kick, strong/weak punch, etc.) with a character specific special move set. Ryu and Ken's Hadouken, Cyrax's cyber net and detonators, etc. Most of the time, speed is not applicable, due to a confined setting. These are just a few of many character unique traits that are absent from a lot of fighters.
All that said, let's recapitulate in an abridged version. Fighting games as we know them adhere to a strict pattern. Differences between characters usually go only as far as special moves, and similar moves with slightly different functions. The dimensions of the game remain confined to close-quarters fisticuffs, which automatically alleviates potential dimensions. It does, ultimately, boil down to outputting more force than your opponent.
The general fighting game archetype has been tweaked here and there, but it has almost never changed. You are confined to an unremarkable arena, in which you're forced to strongarm your opponent in close range. There are just, plain and simply, a lot of restrictions, especially in movement and freedom. Trademarks include what I like to call "dial-a-combo", in which a button sequence results in a combo. Oftentimes, a combo's intended implementation is unrealistic to execute, but its pseudo-complexity belies its unrealistic nature. Killer Instinct Gold is a good example. Fulgore had at least 50 auto-doubles, and 100 combos, but none but one were ever used. Why? Because it was the strongest, and covered the most bases. In other words, just because a game like Soul Calibur has 8,000 different combinations, does not mean it is complex, despite it giving that illusion. If you ever watch an SCIV match, you'll see Hilde trying to do her auto-ring-out and a Yoshimitsu trying to spam her to death.
Overall, some fighters have a lot to offer. But the reality is that many of them are one-in-the-same in many regards. Each one operates on a rock-paper-scissors system, and that system alone. Each one only modifying the archetype slightly.
WHAT MAKES SMASH UNIQUE
Now for the interesting part. As we discussed before, almost every fighter out there gives its own flavor of ice cream, but that's the problem: they're all still ice cream. With Smash, it takes the general concept of implementing an arsenal of moves to defeat your opponent, but removes the restrictions of the fighting game archetype. Let's explore a bit what these restrictions are, and how Smash transcends them to add a whole new level of gameplay.
Let's just make a quick list of unique traits that Smash possesses above other fighters. Forgive me if I miss a few.
-Unique stages
-Completely unique characters
-Counterpicks
-Range and priority playing a huge role due to unique character traits
-Survival and recovery
-Free-roam
-Creativity methodologies
-DI
-Tier Lists
Now, time to delve into each facet.
Unique characters: In most fighters I've ever seen, as mentioned in the previous section, characters differ, but how much? Minor differences do exist, such as a different animation for the same class of move, but they're often too negligible to notice. Smash, again, takes the core concept to the next level by giving the characters differing, well, everything. No single character has a single similar trait aside from clones. Ike is, by all means, the antithesis of Fox. Ike's move implementations are completely alien to what Fox's are. The only similarity is the general direction in which each attack goes. Same goes for every character. These differences include weight class, KO moves, trajectories, framerates, planes, gravitation (falling speed, etc.), size, hitboxes, range, etc. I could go on for hours. The characters are fettered by models only in the sense that they have the same amount of moves and each move corresponds with the direction which is pushed, i.e. forward = fair.
By and large, Smash is Smash primarily due to the vast uniqueness of each and every character. This makes it so a player has to adapt to not only a playstyle, but the character accompanied by it as well. Each one necessitates a new way of thinking and playing. Due to the variables given by the stages and other characters, every match-up is infinitely different and each move interaction has to be known, adding quite a lot of depth to the equation.
Counterpicks: Smash is probably the only fighter in the world that has such a vivid counterpick system. Due to each character transcending uniformity, and every mode of attack being available through any given character, a system consequently arises that feeds one character's strength and the other's weakness in a large way. Sure, there are other fighters with counterpick systems, no doubt, but due to aforementioned reasons, I find it difficult to believe as they're... shall we say "intimate". What I mean by intimate is that each character interacts on the minutest of levels, whereas in many other fighters, counterpicks are based more on countering styles from my experience.
Precision: Hitboxes in Smash come in all shapes and sizes. Their knockback power no different. What these unique hitboxes and priorities create is a rock-paper-scissors system for each move. It adds the dimension of precision and prediction and vivifies them. If your opponent is above you, and his dair ***** your uair, then you find a way around it, or use a less conventional move by being creative.
Creative Methodologies: Again, hearkening back to the uniqueness of each character's moves, Smash allows for something most games do not: attacking out of context, if you will. What I mean by this is since you are granted such freedom of movement, an attack that would normally not be used in a certain context can be used to catch an opponent off-guard. A good, yet simple example is Ganon's fair. Most good Ganon users won't attack with it as it's "intended", which is when facing the opponent. Instead, they will jump behind them, shattering the opponent's expectations, which means they drop their shield and eat fair from behind. Most other fighters, you don't need to anticipate anything other than a high/low/grab attack, since most combat is about-face and full-frontal.
The reality is that in a lot of fighters, since you are fully-frontal, you can't really catch someone off guard with much but cross-ups. "Attacking out of context" isn't really an option, since you're always facing forward and have no stage exploits at your disposal.
Tier Lists: Ever wondered why tier lists are such a big deal? It's because along with Smash's unique gameplay, a unique imbalance gets mixed in. Sure, other games have tier lists, but Smash's is different I believe. I've never seen an entire community get hyped up and get into 10,000+ reply debates on why a character shouldn't be where he/she is in relation to another character. What makes Smash so incredibly fun is overcoming obstacles. Finding ways to circumvent hindrances and use everything in your ability to emerge the victor despite unfavorable odds.